Uhm.. Penny's appartment is significantly smaller. Hers is a 2 room appartment while Sheldon/Leonard's is a 3 room appartment with a significantly larger living room.
So your entire argument falls apart.
However many TV shows do not care about realities of life.. Friends had the girls live in a sizeable appartment in the middle of New York while they were a cook and a waitress (at least in the first few seasons).. not realistic. At least they acknowledged this in the show finale by claiming the appartment was under rent control.
King Of Queens.. a delivery driver and a secretary own a sizeable house and at one point buya brand new car on top of her being a fashion victim and him eating out all the time.
I have no clue what scientists at a university make.. they sure could make much more if they worked for a private company but even then one of them could probably afford the appartment on his own but then a central part of the show wouldn't work.