While not claiming to be any authority on the bread sciences, I’d say that a roll is roughly the size of just the palm, while a bun is closer to the size of the full hand.
A bun also passes the Burger Test: Could you easily fit a burger on this bread? If yes, it’s a bun; if no, it’s a roll.
See I don’t know if it’s a size thing. I think it has to do with accessorizing. If you put a filling in it, or iced it, it’s a bun. But plain it’s just a roll.