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I wouldn't say it shrinks it or stretches it. My experience is it helps the leather mold to your body and adds wrinkles which can pull in the dimensions maybe 1/2 a size at best.
I have found through obsessive experimentation on samples and jackets that most leather will stretch about 1/4" to 3/8" (final dimension after drying) if you work it when wet- goat seems to be the most pliable.
Cheers
Mark