http://img00.deviantart.net/15bd/i/2013/039/8/f/jerry_seinfe...
similarly the main apartment in friends seems pretty large
http://pre14.deviantart.net/3e01/th/pre/i/2016/036/8/8/frien... - note that this diagram is for both the girls apartment and the guys apartment across the hall, so we are looking at two apartments here
quick credit to the artist of these diagrams, they have a ton of other shows on their deviantart page[1]
But why did Seinfeld/Friends engage in this trick?
My guess: Seinfeld/Friends was aimed to appeal to a mass american audience, which is why I imagine they had such large apartments -- they wanted something that still looked "small" to middle americans and outer suburban dwellers, but it still obviously had to be much larger than real manhattan apartment sizes, because filming in a realistic 350sqft apartment set (1) would have been awkward to even shoot (2) would have seemed so small as to be hard to believe or at least steal the scene focus when the apartment is supposed to just be a backdrop.
So yeah, compared to that, the flat they had in "Friends" was incredibly spacious.