What do you think that fat guy did to get him in prison?

I'm guessing it was some sort of accident, like a hit and run. Something he didn't mean to do, but his fear overwhelmed him and got him into more trouble than he was already in, just like what we saw in the movie.

Most likely a petty crime since he did not seem like the type of guy to do something major like what Red was in for, and what Andy got convicted for.

Exactly. I constantly (and correctly) judge people and their crimes based on their mannerisms and looks. Well done......

Kevin Bacons depiction of a pedophile is way more accurate as the stereotype of the typical loner like Toy Story 2s toy collector.

You must be real fun at any parties. And movie dates.

TxMike, since you seem to have missed the point of the question, let me explain. You are right that there is no reason given in the movie and that character exists within the story as a means of fulfilling a narrative purpose within our world. But there is still an "in universe" reason for his being there. When the characters ask each other why they are there, for instance, they admit the crime they committed (Red) or claim they are innocent (everyone else). No one says they are simply there or acts as though any other inmate is simply there. When there is no real world explanation for something within a story that people enjoy, part of the fun is in contemplating what the in universe explanation for those things might be. That was the intent of the original question.

The fat guy was never in prison. It is fiction.

Maybe FatAss is a parallel to Andy. At the end of the movie we learn that Andy was wrongly convicted, but ultimately gets his freedom, and revenge on the prison guards and the Warden. But at the start of the movie, during the "fresh fish" scene, were FatAss is pleading with Captain Hadley, he (FatAss) begs that hes not supposed to be here. Not him. So it is possible that FatAss is another wrongly convicted prisoner but, unlike Andy, never got his freedom.