Some people have drier skin with a good Ph that simply can't hurt a pearl no matter how many years are involved and some people are caustic and can destroy a necklace in a couple of years.
Even with a theory like that I wouldn't see how it would elongate them because it would be gravitational influence that would change the structure. In fact A theory like that would mean that ancient pearls would be flat as a pancake.
Do you mean worn on a leather cloth, or strung on a leather cord?
What is worse with leather, in your opinion (or would it be worse with Tahiti pearls) ?
You probably just thought it was smaller after putting on all that muscle at the gym. Happens to me all the time.I mean strung on a leather cord. I wore a 16 mm peacock for a few years (before leaving it in the locker room at my local gym) and it actually started to shrink a little bit.
I'm not sure I understand the second part of your question.