For years now, my thumb shrinks while bowling.

At the start, I cannot get into the thumbhole in any of my balls ---which I DO have drilled for a snug fit, but eventually after gradually working it in, or using various techniques (the dollar bill method, for instance) I am in, and ready to bowl comfortably.  Usually good for the rest of the night, though marathon sessions sometimes require the removal of a piece of tape.  But mostly, it's only a problem for a few minutes before bowling starts.

But my question is not about combating or correcting the problem. I'm just curious as to why. In other words...  it seems almost like a magic trick that I am able to eventually fit into the same thumbhole that is obviously way too small at the start.  

Thanks in advance for any explanation