I don't know if I'll explain myself properly, but Yabusele, more than being fat or thick, has a peculiar body morphology. Going into detail, it has to do with his butt (sorry if it's a bad word). There are some players that has this part of the body seemingly more separated from the upper part, more rounded.
That's what happens with Yabusele. Looking at him from his front part, he doesn't seem thick at all, but the problem is that you watch him mainly running (the master camera is on one side of the court), and you are inclined to see him fat as a whole, though he isn't. He has more distance from his belly to his bottom than a normal physionomy (Anthony Mason is another good example), but he can't do nothing about it. Sully or Glen Davis were chubby, Yabusele isn't. It's just his body.