Too much bevel will shorten the span.
Too much bevel and it can feel like you have a shortened span because the proper support for your thumb is there. The top of your thumb could be closer, DEPENDING ON WHERE THE BEVEL IS ON THE THUMB HOLE. The lower part of your thumb will still be in the same position.
Too little you can get rubbing at the base of the thumb.
Too little bevel and you may not get your thumb all the way into the hole. You MAY get rubbing, again, depending on where the bevel is on the thumb hole. It is not in the same place for every one.
Theoretically though if everything was just right and you release correctly, you wouldn't need any bevel.
If everyone's thumb was exactly the same shape and all the bones, ligaments and tendons and webbing (between thumb and index finger) were all the same size and in the same location, THEN no one would need any bevel.
Some people need more AND different beveling than others do. Some people fee more comfortable with different amounts of beveling. Not everyone is the same in needs, feeling, wants, desires and hand strength.