i think a lot of this comes down to poor sex education and the lack of true sexual expression in our society. a lot of men are almost conditioned to think about sex solely in terms of they themselves getting off and think size or some "trick move" is going to be enough for the woman. and a lot of women kind of get used to that so just lie there and hope maybe the guy'll trip over an orgasm for her.
more than size, it comes down to skill and consideration for your partner...on BOTH sides. you have to have the right attitude going into sex. i always go in with the notion that i don't care whether i cum or not. because i know i probably will and if not i can just go jerk off later. so my focus is primarily on trying to make her feel as good as possible and having a lot of fun. that's all that should count, really.
my last girlfriend had a lot of problems opening up sexually at first because her previous boyfriend from high school was apparently very small, not good at all and didn't like going down on her. so she was used to that sort of "lie there and be the receptacle" attitude and was thrown off by my "hey, let's try this position" and "may i please worship between your thighs for the next half hour" attitude. admittedly, i was thrown off by hers as well and pushed her a bit too hard at first to break out of that kind of thinking...because i didn't understand her mentality or perspective at the time. once we kind of got our vibe down and started opening up to each other more, the sex (which was already great, even with our problems) just got exponentially more intense and fun, especially after i gave her her first orgasm (ever!). suddenly, we were making up positions, doing it all the time everywhere we could...she starts talking about role playing and filming ourselves and doing it outside (at which point i wondered exactly how she tripped upon my list of fantasies.
). apparently, her new boyfriend isn't AS sexually attentive (or from what she says skilled...which both flatters and depresses me, lol), but she's much more aware of what she wants and likes sexually and doesn't need to "just lie there" anymore.
so i'd say on the one hand, definitely look at your technique and if there's anything YOU can do to improve. and then...consider that some women have just gotten used to lying there while the sweaty bastard they're with tries to do his best impression of a jackhammer. so be patient with them, actually TALK to them about what they want, what they like, what they need. believe me, not only will the sex be better for her, but if you focus on her it gets infinitely better for you as well.
my two shillings.