Actually, the 737 and 757 (and 707, 720,and 727) have the same cross section. What matters is seat pitch. That's how many rows in a given length. The airline determines that, not the manufacturer. And they can change it easily. I've seen some of the 737NGs that were quite comfortable. I've also been on 757s that were cattle cars. As a retired airline pilot, I often shook my head at some of the decisions as to how many seats a plane should hold.
Lew