Ok first lets make sure we are talking about the same thing.
A Thumb slug is a round solid piece of vinyl or urethane they hammer into a hole in the ball, then drill your thumb hole in.
See picture (Yours may be a different color.)
If this shrank it would pull away from the ball coverstock and be loose and likely come out. Plus it is unlikely that it's absorbing oil and expanding.
As for the ball going faster on dry lanes, I think your just throwing it faster to over come the dry lane and don't realize it.
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