Pretty sure the vast majority of the consumption is young males, especially given that older guys are much more likely to have gotten married and/or have jobs & families and basically not have time for that stuff (general worldwide trend as well - porn consumption decreases as you get older). Furthermore Japan's rate of obesity is actually not that bad, so the direct character insert wouldn't usually apply either because very few of the consumers reach those levels of health. Remember that anime/manga culture makes it seem like half of the population are otaku NEETs, when instead many of the consumers are normally functioning members of society.

If I had to guess, it's a morale/ego thing. The men are designed to be unattractive relative to everyone. I don't know a single person in real life who is that fat and that unattractive, and while many people might have flaws, lined up with the doujin guys almost anyone could feel validated. So when you look at a doujin and say, "wow, that guy's that ugly and still getting hot chicks - then even someone like me could do it!" and feel a bit better about yourself. Whether it actually works or not, I have no idea. That's my understanding of it.