Originally Posted by ATOBTTR

Also from Airliners.net:
The 737 and 757 have the same cabin width but the floor is lower on the 757, putting more height between the floor and overhead bins. This also impacts where ones head and shoulders are relative to the cabin walls when sitting based on the curvature of the fuselage which is why the 757 often feels roomier and more spacious than a 737 - because in the space and dimensions that matter, the 757 is roomier (or rather provides more usable space for passengers).