If the girl is on the top while having sex, does she do anything or does he do the work?

I was reading posts and noticed that a lot of them said how sex should be easy if the girl is turned on and relaxed. I have a smaller build and perceive it to be proportional in "all areas". I'm 19. I really enjoy sex and I have been sexually active for two years. However, I have found that no matter how much foreplay my boyfriend and I engage in, sex is difficult to start with and I feel sore during the end and afterwards. My boyfriend has also noticed a large difference in "tightness" between myself and other girls he has been with. We always use a good amount of lubrication and I do feel really relaxed and aroused with him. I don't understand how a guy can have a larger or smaller penis, but yet everything I have read on your site says that women have little to no variation in size. Does this mean that I'm generally the same size as the girls he's been with and that there is something else wrong with me?