I like my thumb holes a little bit loose. If thumb hole is slightly tight starts to rub and before you know it you got blister. By the time you noticed the blister it has torn the skin, and now you have a problem and it will effect your bowling. Its a lot easier and quicker to make a large hole smaller with tape.

I try to go about 1/8-1/16 too big. That gives me plenty of room for tape. I have never been an advocate of a really tight thumbhole though. If the thumb is too tight it will be almost impossible to get it off of your thumb.
Think of it like this......there will be hardly any if any at all air allowed to go in, creating a suction.
This was my thought anyways.

I know of guys who drill a thumb hole way big and then use 15 or 20 pieces of tape to shape the hole. My personal opinion is that is nuts.

Most generally this is done by “proshops” who don’t want to take the time to shape a thumb into an oval. So they drill it big so the bowler will not stick and tell them to tape it.

A thumb can be snug if the pitch is correct and get a clean release. This reduces squeezing the ball, makes the ball feel lighter, and makes the release automatic.

The thumb hole should be reasonably tight when your thumb is at it’s largest size. When your thumb shrinks then you will have to use tape.

I personally like the feel of tape. My thumbs are setup where when my thumb is it’s biggest I still need 1 piece of tape. When the thumb is small I may need 5 or 6 pieces.

99 times out of 100 if you stick in a ball it is because of the thumb. In fact I don’t believe I have ever seen a person stick in the fingers if their thumb released and the only way that is possible is if the fingers are inserted to far (past the 1st knuckle) on a finger tip ball.  

As a side light almost all bowlers are afraid of sticking in the ball but very few are afraid of dropping the ball. I see 100 bad shots because of dropping the ball for every bad shot for sticking.