Or maybe it is only a psychological process to make you think that the girl involved is smaller that she actually is due to the fat guy.
Think about it, oh and while you're thinking about it, I'm gonna give you an example. But I'm warning you, some people might find this example "racist" or "wtf why only man dicks?" oh well I'll put it in spoiler so it is not for the easily offended (but really if you're easily offended, stop coming on Fakku, lol)
Imagine there are 5 man in a room, THEY ARE NAKED. There are four white guys and one black. The black guy will look like he has a really big dick. But if you take the same room, same naked black man with his dick, but instead of 4 white men, you put 4 Tauros (yeah the pokémon, those pokémon happen to have a really big dick didn't you know). Well happens that the 4 tauros are really excited, the black man, even if he got a 15 inches long penis, his genitalia will look like a small wiener compared to those. I hope you understand my point now. No offense black man, I love you guys too!