I think he's using "fat" and an analogy for "lazy". Even though fat people aren't always lazy, and lazy people aren't always fat, it's a common conception and used to induce a stereotype.
Or maybe he's talking about the physical exercise a pirate gets from pirating. You might think that just because a pirate is sitting in one place for ages, he mustn't be moving much and so he's not burning off the food he eats, and accumulating fat instead. But then... even if his ship was moving at top speed, he still wouldn't be moving. In fact, no pilot really "moves" when they are piloting a ship. So are all spaceship pilots fat, then, by this logic? Except maybe capital ship pilots and some larger transporters, they have a bridge they can move around.
Unless you were using "fat" as an analogy for "greed" which is an all consuming desire for more possessions that one would require. Which would also be false, a lot of pirates are trying to make a living (in-RP), and piracy is the only way to make ends meet (just like Somalian pirates). And Fat people aren't greedy, anyway, they're gluttonous, which is the desire to consume more than one requires. I don't think you can accuse Corsairs of that.