Your pictures make you look very decent sized.
My guess is
1) She's not that into you
2) She's just not that into sex.
3) She's a "dead fish" (might like sex but thinks she's just supposed to lie there motionless)
4) You're not good in bed (and that has to do with a lot of factors, size is only one of them).
If the other girls you've slept with have all been eager to have sex with you, then it's probably one of the first 3. If this is common for you, then you probably need to learn some skills. All in all, it sounds like it's a mix of all 4 though, since "hammering it in" is considered not so great in bed.
@njman2008: Not just you. I don't think I could enjoy it much if it looked like she wasn't enjoying it.
Still though, your comment was pretty absurd. If he's ok with fucking her and she's ok with fucking him, who are you to judge? Even though the sex isn't ideal at all, it's still sex and maybe HE prefers sex with a bad in bed chick than masturbating. That doesn't make him a shithead unless he's forcing her to fuck him.