I remember from somewhere, that Superman's cousin acquired powers faster than Superman himself. Would that mean that Superman is just as strong as any other Krytonian if they were still around in the same environment? Is Superman really super compared to other Kryptonians?
Yes, Superman was really super even compared to other Kryptonians. (Or at least he was until the DCnU New 52 changed the DC Universe, yet again.) Under the original premise, as the Last Son of Krypton, Kal-El/Superman gained his powers due to the slow and long-term exposure of his body to the radiation of our yellow sun. This made him a force to be reckoned with in comparison to other Kryptonians because of his much greater strength, durability, experience and control in the use of his many powers.
Superman and the rays of the Sun
Yes, according to writers who wanted to boost Superman's powers in the early 1950's he gained extraordinary abilities after his exposure to the Earth's yellow sun. Early Superman was only capable of leaping a quarter of a mile and running faster than a freight train. These modest feats would still make him amazing by the standards of the early 1940-1950s. Take a look at these clips from Secret Origins #1.
Early Superman being heroic, leaping tall buildings and chasing cars on foot.
No, there is nothing currently in science that would explain this, so don't think too deeply, the writers who conceived of Superman's powers beyond those that were derived from Earth's lessened gravity.
They knew next to nothing about stars, solar energy, radiation or properties of stars.
They did not realize how little difference there is between a red sun and a yellow one besides color, energy output and the star's life-span.
This exposure to a yellow sun would activate a process by which his Kryptonian cells would become super-charged and along with the reduced gravity of Earth, make him capable of a host of amazing superhuman abilities, making him one of the physically most power powerful beings on the planet.
That said, over the years, there have been many reboots and changes to the premise of how quickly Kal-El would gain his powers (one continuity had him gain his powers as a child and have an entire career of superherodom as Superboy.) This was later retconned out of existence. Most tales now have Superman gaining his powers as an adult after years of exposure to solar radiation.
This however does not explain the rapid speed that most Kryptonians seem to gain their powers once exposed to the yellow sun of Earth. In the new Krypton stories, the Kryptonians also possess superpowers and this was not after decades of exposure.
So what makes Superman super?
What make Superman super has been characterized by the RESTRAINT he has in the use of his powers. Even when Kryptonians have had powers from the sun, he was often considered stronger due to the amount of exposure he had compared to theirs. Current DCnU seems to be working toward the idea of Kryptonians needing time to gain powers under a yellow sun. Supergirl's spaceship had a device designed to charge her cells with yellow sun radiation.
In earlier continuities, Superman was considered to have a store of internalized energy that had built up over the decades, making him stronger, tougher, with greater endurance than a Kryptonian who found themselves empowered by the sun but not with the longevity Superman would have had being continuously exposed to the power-giving radiations.
His other advantage, played up by writers was his experience with using his superpowers. This may not be as apparent with the reboot of the DCnU but in earlier continuities, his experience being Superman, the use of his powers, the vulnerabilities he had to learn to overcome gave him a decided advantage in a fight with other Kryptonians.
Imagine discovering yourself with the ability to hear all along the auditory range and not having practice discerning which sounds were important, how to differentiate sound levels, and controlling what ranges you were interested in hearing sounds at.
Supergirl is overwhelmed by her new super-senses. Supergirl #2
Now add to that visual acuity through the entire electromagnetic spectrum, telescopic, microscopic visions, energy projection from your eyes, a sense of smell ten thousand times better than a human and all of this happening simultaneously in addition to having strength millions of times greater than you previously had before, all of this would make for a confusing time as you learned to adjust. Did I mention, flying?
In the previous DCU all Kryptonians under a yellow sun, possess vast superhuman strength, speed, and stamina, invulnerability; flight; super breath; x-ray vision; telescopic and microscopic vision; freeze breath; heat vision; and super hearing. If they are trained properly, they will be able to use these abilities with some degree of effectiveness.
Why does Supergirl in the DCnU appear more powerful than Superman?
If previous continuities are any gauge, Kryptonians grow more powerful, the longer they remain under a yellow sun. So both Superman and Supergirl, assuming they do not deplete their store of internal energy (See Doomsday vs Superman in the Death of Superman) will continue to slowly grow more powerful over time.
- Supergirl has appeared in conflicts with Superman to be more physically powerful. Superman asserts he has spent his time as a superhuman learning how to restrain his powers to do as little collateral damage as possible.
Exercising restraint often means having to take significant abuse on Superman's part.
Where Supergirl, new to the fragility of the Earth's environment has not yet learned how to control the amount of damage she causes when she uses her powers at their highest settings.
If this does not change, she will most assuredly kill someone, most likely as an unintended consequence of using her powers without a degree of discretion.
What makes Superman Super (that is, extraordinary) is not merely the set of powers he has as a Kryptonian under a yellow sun. Those powers are standard for any Kryptonians on Earth. What makes him special is his personality. The way he uses those powers for good is what makes him exceptional. The story Red Sun is all about this, as it has Superman's ship land in the USSR instead of Kansas, so he's raised by Soviets instead of the Kents.
But purely in terms of powers, he has no abnormal powers for a Kryptonian. Any other Kryptonian on Earth would gain powers roughly identical to Superman's. Admittedly the varying physical strength of various Kryptonians would still apply. So Supergirl would be physically weaker than Superman, simply due to her smaller size and lower muscle mass.
It's unclear from the question what canon or timeframe we should be pulling from here, but in pre-New 52 DC Universe, Zod and friends escaped from the Phantom Zone and were physical equals to Superman. Later, the shrunken bottle city of Kandor was embiggened, which released 100,000 Kryptonians on Earth, and they were roughly as strong as Superman. Superman maintained an advantage over them in combat due to his greater experience with his powers. The other Kryptonians were used to their standard (roughly human-level) level of strength/speed/etc when they received their powers, which took some adjustment.
I think the reason for other Kryptonians gaining their powers relatively quickly is that planet Krypton possesses conditions that are naturally more challenging that Earth, e.g. higher gravity, a less nourishing atmosphere and higher air pressure etc. Their bodies were naturally stronger and tougher upon arriving on Earth, providing intrinsically elevated physical abilities prior to yellow sunlight exposure.
This may be why Kara seems stronger than Kal in so many iterations of the canon. However, I believe that Kal has a different advantage in that the amount of time spent on Earth, particularly in growing up under its yellow rays, has developed his ability to absorb radiation more efficiently. This therefore allows him to perform greater feats at full strength, making him unique and, ultimately, the most powerful 'superman'.
(Note: I'm watching Man of Steel at the moment so this explanation is somewhat influenced by aspects of the movie's more grounded logic, which brings some interesting theories to the table.)
FWIW, as memory serves, Kal-El had been designated by OA to become the ultimate Green Lantern for reasons I don't remember anymore. Whether this was genetic, or attitude, I don't know. There is at least one instance where he gets the ring though, in the Elseworld "In Darkest Knight" where Bruce Wayne gets the ring instead of Hal Jordan. Which, when you think about it, makes a heck of a lot more sense than Jordan. But OA gets scared of how powerful Green-Wayne is, and gives rings to Clark, Diane, and I think Barry Allan to take Green-Wayne down. I haven't read it in a long time though, and as memory serves Green-Wayne wipes the floor with them.
What makes superman special among kryptonions is actually pretty simple. It's the fact that he's a natural birth, it's been said by multiple sources that him being a natural birth allows his body to process solar energy better. Because he wasn't genetically changed and is in fact a more pure kryptonion that the others such as his cousin he us more like the kryptonions of the past who were know for conquering worlds before they became a more scientific culture. The reason why at times Kara has seemed more powerful is due to the fact that due to his need to fit in among humans, kal has been putting barriers between him and his powers since he got them. He would have to work on unlocking them throughout his life hence the reason it took him so long to fly while other kryptonions caught onto it pretty soon. Another reason is that Kara doesn't have the restraint that Kal does, which is something she struggles with throughout her time as a hero.
Actually some comics state Supergirl's cells are more efficient at consuming the solar radiation of our sun. As such her potential is said to be beyond that of Kal El. That being said he has more experience. And in modern comics Superman is said to be more powerful than all other Kryptonians because he is the reincarnation of the Kryptonian Sun God Rao.
What people forgot to mention is that Superman absorbs and stores solar energy more efficiently than other Kryptonians. He was the only living natural born Kryptonian. All other Kryptonians were genetically engineered to fit a specific role within society both physically and mentally. So Kal-El's body was that of a pure Kryptonian as opposed to the rest of the population that had had genes removed or spliced or subdued for their specific roles. This gives him more power and free will due to his genes not being altered or mental programing installed in him as opposed to other Kryptonians. But he works so hard at restraint to both not do harm and be human that he looks weaker in initial encounters because he holds back until he gets a feel for how hard he can go.