Your thumb and fingers not only swell with humidity but also with body fluid retention, like when you drink more beer than you're used to drinking or you eat more salty foods.
Your thumb hole has to be as large as your thumb will ever get. If you happen to have to bowl early in the morning, for a tournament or a make up session, you'll also find your fingers larger than they are in the evening, when most of us bowl. Try putting your fingers in the holes right after you wake up in the morning. If you've never done that before, I think you'll be shocked.
I have a make-up session on Sunday, April 7th, at 10 AM, when we normally bowl at 6:20 PM. I expect to have to take out 2 to 4 pieces of 1" ape from my thumb holes!!!