My sister was in a similar situation. Her boyfriend had a thing for stockings, and she found a folder on his computer filled with pics of girls in stockings/pantyhose/tights whatever you call them, pictures which were mostly from facebook of girls she knew (some they both knew).
I get the idea though. Although I don't watch much porn now, when I occasionally do I much prefer to find some erotic photos or look at the amateur section than all that styled stuff (even though 'amateur' isn't really amateur, but it's styled to look like a homevideo at the very least).
Even when I get inspired for hair, clothes, travel or whatnot I find it more fun to look at stylish pictures of girls on facebook than strangers on fashion blogs. I mean if someone adds a bikini photo on facebook, we all click on it right?
It sounds like that's part of what's bothering you. These girls aren't just some random porn stars or swimsuit models or celebrities he'll never encounter. There are always going to someone slightly hotter out there, but being real is obviously the greatest advantage a regular girl has over a celebrity. Otherwise girls would be freaking out over hot celebrity women all the time.
If you're not happy with your body, change it. Seriously. I'm not saying that's the entire solution here, but if most girls you know have better figures and even the idea that your boyfriend is jerking off to hot women (which all men do...) bothers you so much and makes you feel undesirable, the root of it is probably that you don't feel attractive enough to compete with anyone. Take more care of yourself, go on a diet, work out, whatever you need. You should be proud of your body, not just "ok" with it.
I have to agree with the others that the snooping isn't good. I have never actually used a guy's computer (have my own after all..) but I wouldn't snoop and I would be extremely pissed off a guy snooped through my stuff. Not because I have anything to hide, but it would really lower my opinion of him. He'd also look jealous and desperate.