I had a similar issue, except my thumb doesn't swell. My thumb hole was too tight and I had all my balls resized to make it bigger. Now I think it's better to have it bigger than smaller, because you could easily add bowlers tape to make it more snug. If it swells then it'll feel just right. What I do when I don't have bowler's tape is press the inside of my thumb to the ball...on the upswing it tightens up, but on the downswing it releases just right before my fingers to give me control. So far that works for my consistency.
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