I had an ex of about 5'3 who was pretty big (~170lbs). We sometimes had trouble with penetration (I'm 8" x 5.5"), but if I recall correctly after that it went pretty smooth and she enjoyed it a lot (though at times she would bleed, so I guess I tore her open then or something).
Another ex was a chinese girl who was like 5'1 and ~120lbs. She wasn't one of those slender Asian types, though she wasn't fat at all either (but relatively had pretty wide hips and large breasts, though you'd consider them small for non-Asians
). But with her I had a lot of trouble. Preferably she only did it on top and that was in any case about the only position where I was able to penetrate her (which took a few minutes). I don't know, she really loved me so she didn't mind, but I had the feeling she never really enjoyed the sex (just the body to body contact).
The thing was though, the only trouble she really had was the penetration. With some practicing and experience I could get it in all the way and that was no (extra) trouble/pain for her. So just the beginning of her vagina, where it gets penetrated, hurted for her.
My current gf is 5'8 and ~140lbs and had pretty wide hips and large breasts. She was a virgin but penetration was a piece of cake, so I never had any trouble with her and she really likes my size.
One position though we've experienced does pain her, and that's when she's lying on her back with her legs lifted up. Then I go too deep or something. Does anyone else have this experience as well?
I know porn isn't the true world, but I see much bigger dicks there go into much smallers girls, and hard! (and seemingly without a problem)