The reason? Female hormones. Men with excess fat showed higher levels of the female estradiol sex hormone. This substance apparently disrupted their bodies' natural "male" neurotransmitter chemicals and slowed their progression towards orgasm. Ironically, the less masculine their bodies appeared, the better lovers they proved to be.
Better, meaning taking longer to come, I suppose. Well that's one metric, I guess. Not the most apposite one for me, but then I do not have to deal with the vast chasm between male and female sexuality. There are times when I feel a kind of pity for heterosexuals. The mismatch between men and women can seem so ludicrously great at times, however much effort is put into describing this as "complementarity".