Jor-El says in Superman I that Kal-El will not be like the humans of Earth. That he will have super strength and that he will be much denser than a man. Being denser means you have more mass which means you are heavy relatively speaking compared to a man. How heavy is Clark then as he is walking around? I never noticed cars he got into dragging on the ground or anyone saying he was made from carbon nano-tubes.
Lois Lane interviewed Supes in the movie Superman. Excerpt:
LOIS: And how big are you? How tall are you?
SUPERMAN: About six four.
LOIS: And, uh, how much do you weight?
SUPERMAN: Around two twenty-five.
It's possible that since Superman can counteract the force of gravity with his power, his weight might be one that he assumes to avoid damaging people or property that might collide with his real and presumed higher mass. But in the movie Superman Returns, Superman is rendered unconscious by extreme exertion and exposure to green kryptonite. During his unconsciousness Superman is moved on a gurney with expected effort by ordinary humans and laid upon a hospital bed, which does not collapse under his weight. If we assume that Superman is not levitating when unconscious, then his mass cannot be more than a few hundred kilos else he would have destroyed his hospital bed and gurney.
"Much Denser Than a Man"
While density refers to mass/volume, it could be that he was referring to something on a different scale than you were thinking. Density normally means mass/volume. However Population Density doesn't refer to the mass of all the humans per section of volume. That would be stupid. It refers to the # of people per area. Completely different. So my thought not fact, just a plausible explanation Jor-El could have been referring to a protein density, or a cellular density. If this is true then Cal-El, or Clark Kent, or Superman is made up of lighter-weight, closer packed, (and obviously stronger inter-bonded) make up of cells and proteins. That way he's "denser" but he's not heavier.
Number of cells/proteins per volume rather than mass/volume. #IMJUSSAYIN!
The gravity force of Krypton is an important data here. If we want to measure the physical strength of a humanoid from another planet we need to examine how gravity force will affect it. A far greater gravity force will require human-like bodies to have denser bones and muscles in order to avoid being smashed by it, this heavier body building will surely be a big part in Jor-El superhuman strength.
We can only assume a certain mathematical correlation between gravity and body density, however, it could serve as an approximation to the actual mass of superman on Earth. If we take a weight of 220 lbs. as something plausible for a muscular 6'4'' earth man, it would be feasible that one humanoid of same size from a planet with twice as gravity force as the Earth would weight about 440-450 lbs.
I've not found references to Krypton's gravity force, but Wikipedia suggest it as "massive" comparatively to the Earth, wich suggest that Superman would probably weight several metric tons on Earth (and it would easily explain why he's so strong and thought).
Why he lies about his weight and why the movies and comics fail to represent the fact of such a weight is only because neither movies nor comics pretends (or can) be exhaustively correct at a physics level, for both, narration and resources reasons.
maybe he is atomically denser but composed primarily of undiscovered elements? or; as he already emits anti-gravitons perhaps they are somehow lattice bound into his atomic structure? or to go realllllyyyy meta perhaps kryptonians employ some unknown quantum principle involving gluons and photons somehow being transformed automatically into gravitons by kryptonian cells and their brain chemistry simply allows them to control the extent to which this takes place, enabling flight (this would make them much more atomically dense without being to much heavier than an average human as their cells may be able to independently regulate their own mass given a specific gravitational field) although this would give rise to many more problems for the cloning storylines... no evidence to support this just one more theory to add to this list...
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Density - a measure of the amount of information on a storage medium
The human storage medium for a single cell is DNA. Two strands in a twisted bond of 26 chromosomes.
Superman has more. How much? Well enough to be considered "denser" than humans. So much more, a single cell of his body has the capacity to process the suns energy into radiant energy that his body can:
Heat vision - Focus energy to a single point and then project it out.
create a shield of force around his body that can be extended slightly beyond its periphery...like a suit of armor. Force field that he can contract or expand and seem like he has super strength.
fly - warp local space to create gravity
freeze breath - draw energy from the air to cool it and funnel it in a specific direction (opposite of heat vision)
Super hearing - filter out sounds or magnify others
Basically, his body is normal but it's the manipulation of the the suns energy that allows him his "gifts". The problem is Superman isn't into researching what his body can do. He just uses it like a hammer instead of treating it like a fine machine.
At least, that's how I would do it.
Kal El radiates this energy around him creating a protected environment as he is alien to earth's atmosphere and needs to filter it. His radiant environment filters out viruses, bacteria, fungus and anything else that would harm him. That is why he was constantly sick as a child. It wasn't until puberty began that his body began to create the protected environment as it is now. It is also why kryptonite is bad for him. It absorbs that radiant energy he gives off, converts it and then radiates it back out. Which does havoc to his biological systems.
Want to remove that power from superman? Simple, convert his "DNA' or whatever to a version that does not convert the sun energy to radiant energy. Then he would be...
just another man.
I thought Joe and Jerry explained all this?