As far as I am concerned, a thumb hole should be tight enough that you almost have to force your thumb into it and you should be able to relax somewhat and still be able to hold onto to it without exerting any "excessive" pressure.

 

The thumb hole should still be large enough to be able to put your thumb into it when it is at its maximum size, due to swelling from ANY cause whatsoever. When your thumb is smaller than maximum, tape, white or black should be used to decrease the size of the hole based  on where and how you feel most comfortable, using black or white tape, in the back of the front of the hole.

 

FYI If the thumb is loose when you just place your thumb into the hole, it will be way too loose when you swing the ball with force in a normal delivery and it will fall off UNLESS you use excessive pressure to hold onto it.