Well, I’ve been heavy (300+ pounds at 5 feet 6, wearing a size 24-26) and I’ve been “normal” (165, wearing a size 12-14), never been slender. I’ve had partners who were thin, average, and heavier than me.
I’ll try to respond to your questions without getting prickly about the implication that sex with a “obese” person is somehow in a different category from “normal” sex. I felt an undercurrent there, but I’m hoping none was intended.
The biggest contrast (to generalize greatly), is that when I’m fitter with a fitter partner, the sex tends to be more atheletic, more energetic, and more rapidly diverse in terms of changing positions. When either partner is significantly heavy, the sex has tended to be more leisurely, with changing positions a more thoughtful and sometimes cumbersome process. “Heavy” sex tended to involve more foreplay, manual and oral stimulation, and less “banging away” penis-in-vagina time. Heavy sex tended to be more comfortble, less hurried. I felt more valued as an individual and less as an object (meaning an objectively sexy person outside of the relationship).
I’ve also found that the heavier partner tended to initiate sex more often–needing reassurance that they are sexy. I realize that the common assumption is that women tend to feel unattractive when heavy and so avoid sex. It never worked that way for me. Sex helped me feel desirable and I liked that. Of course, I didn’t stick with any partner who found me unattractive.
I’ve never had any trouble getting the “naughty bits” together, even with two of us well over 250. I would say that there is usually a tendency to be shy about nudity in the early stages of a sexual relationship where one of more partner is heavy. When I was heavy I tended to want to “cover up more” or wear sexy garments rather than having naked sex. I liked to have my breasts supported.
I guess I’ve always been puzzled by people who find sexual pleasure to be related to the “societally defined” attractiveness of their partner. Perhaps my attitude is unusual. I’ve never sought sexual partners for their “objective” attractiveness, but rather because I enjoyed their company. The sex wasn’t better because he had a handsome face and good muscular back. The sex was better because of his responsiveness, his attention to my needs, our mutual chemistry, an adventurous spirit, a sense of humor, etc.
So much depends on the mental attitudes of the participants. A heavy woman or man with “presence” is as sexy (in my opinion) as a more fit individual. I’ve felt incredibly attractive and remarkably sexy at my heaviest, and felt criticized and unattractive (and therefore un-sexy) at my fittest. No matter how heavy I’ve been, I’ve always found ways to “put on my sexy persona,” dress attractively, and walk into a room with the attitude that “I am sexy and any man here would be lucky to have me.” Seems to work like a charm.
I hope that’s a start. Sex is fun and fullfilling at all sizes (IMHO).