They can change (based on what, I'm not sure, you can check
the manual), but you can edit it if you like. You probably need to be in commissioner mode, but after that from the team menu select settings then look for market share.
I'd say the Mets market share in 1962 was probably nowhere near the Yankees, regardless of the population of New York, but I would say it was most likely higher than whatever the smallest in the league was. An expansion team almost anywhere would probably have a larger market size than the smallest in the league for the simple reason that baseball fans would be excited to have their own team, but after a few years all bets would be off as the newness would wear off.