This is a great fat girl-lover topic. The sweaty, stinky fat girl stereotype is not entirely unfounded, especially from the back.
Been with skinny girls and fat girls and everything in between. Fat girls get sweaty and steamy under their apron bellies, in their rolls, in their belly button, in their fupa/crotch, in their ass, etc., just by existing in their obesity and having no air circulating in between folds of fat. Fat girls end up developing scents and smells that skinnier girls just don't. And it doesn't matter how good the hygiene is; being fat just makes things worse. For example, if your fat girlfriend has been lounging in yoga pants or leggings all day (i.e., she hasn't showered since the morning or the day before) or you take that drunk fat girl home who has been sweating it up dancing in the bar or club, then you're in for the real fat girl experience. Especially when you do it from behind.
Unlike skinny/normal sized girls, every single fat girl I've banged from behind, without exception, has had a strong whiff of swamp ass or musk coming from that ass that just jumps into your nostrils during each thrust in doggy (save for situations when they are fresh out of the shower). That smell from behind initially catches you off guard when you first spread those fat cheeks and put in it. But then you get in the rhythm and either get used to it / don't mind it or end up welcoming the smell (if only because it reminds you of how fat they are). For the non-virgin, fat fetishists on this board, you know what I'm talking about.
The smells and scents are probably why, among other reasons, normal dudes find sex with fat girls disgusting. Fat girls are definitely an acquired taste . . . err scent.