707, 727, 737, and 757 all have the exact same cross section. One of the advantages of the 757 is that it sits higher off the ground and can accommodate the large engines necessary for a longer fuselage. More generally, 757 is designed to be longer, with larger engines and wings, and higher fuel capacity. 737 had to have these features bolted on as it was stretched, fitted with higher bypass (more fuel efficient) engines, and given larger wings and fuel capacity.