Rewatching the film and reading through the wikis I'm a bit confused about the Kelvin. When comparing them, the Kelvin seems smaller (but seems to have a similar sized shuttlebay) and better armed.
Is that correct or am I mistaken?
The excellent Ex Astris Scientia website offers a wealth of detail about the various ships seen in the Star Trek universe.
The USS Kelvin
It's worth noting that there's some controversy over the size of the Enterprise as seen in the film, largely because of the liberties taken by the filmmakers in creating such a vast shuttlebay for the iconic landing scene.
Memory Alpha offers an overview of the armament of each vessel
Don't let the relative similarity in terms of armament numbers fool you. The Enteprise seems to be capable of rapidly-firing its phasers on a nearly continuous basis for a whole minute and unleashing photons in very large numbers.