Those dumps are unrealistically large. On their salaries they would be in much smaller apartments or not in the city at all. The TV people know that viewers would be uncomfortable watching the characters living in realistically sized New York apartments so they take liberties with that aspect of the show.
Alan
Their apartments are far from dumps. And I live in NYC (and am certainly not independently wealthy) and my apartment is just fine,as are everyone else's I know. Granted, Jerry's apartment is probably bigger than most stand up comedians would be able to afford. :) --Steph