My true concern is your shame about your body; your fear that he'll judge the "rolls" he already knows you have and probably loves. You're not alone with your worries, though. Sadly, most women carry deep insecurities about their shape and size, thanks in part to the popular media and the "thin" mantra drummed into our heads from the time we're toddlers. Even though one man’s approval won't cure decades worth of brutal social conditioning, talking about your feelings with your special guy should help. Clearly, he finds you hot as dynamite or he wouldn’t keep coming back for more luscious loving, so his responses are likely to take the edge off your concerns. But just to be sure, give him the words you need to hear by saying, "I really want to party on top, but I’m so self-conscious about my body that I need your reassurance first. Tell me how much you’d love watching me up there, and I'll be ready to ride."
I also suggest you view my educational video devoted to this oh-so-popular cowgirl position. It will give you a few new tricks to add to your skill-set, along with even more suggestions for making body image a non-issue while you're in the saddle. Happy trails to you!