Compare the 737 to the Embrarer 175 - the Embrarer is a much smaller plane but it has an elliptical fuselage so that you can stand up comfortably in it and also not get a crick in your neck from being inside a too-small circle.
Many words have been written on the flying experience getting worse, but not enough have been written on the 737. Not too long ago you could find a decent plane like a 757 on a major route in the U.S. (say JFK to LAX) but now you have to squeeze yourself into a 737 sardine can, breathe toxic fumes, etc. It's just awful. But you only hear about it when they are hell-bent on re-engining a plane which isn't tall enough to fit a high-bypass engine and then they have to do all kinds of crazy things to make it fit.
Boeing had a chance to do a clean-sheet design of the 737 but they didn't and now Embrarer is going to eat their lunch.