I've always thought it was like ten times more about length than width. It takes a lot of "practice" before someone can stick their richard all the way inside your butt without it hurting.
the human star fish is only ~5 inches deep before the first bend, and since it's not designed for things to go in pretty much anything is fair game to cause damage
on the other hand it can be widened like any other orifice with some work, so it really is more of a more of a problem of thickness than depth