There are a few reasons that I know of: Heavy bodies are more difficult to move, fat bodies take longer to dissect, and large bodies don’t always fit on the (usually decades-old) tables.
I think that weight is a disqualification that’s been changing in recent years, because even in Europe bodies are starting to trend larger. But it’s still something to keep in mind if you live in like Mississippi and think the University of Mississippi will take your 350 pound grandpa when he passes on.
I agree with you, I was just sharing what I knew of the process. My grandpa was rejected in a different state, but they were kind enough to cremate him for us.