The size of the U.S. boats was dictated by function. You needed a big boat to carry sufficient fuel for Pacific operations, which generally involved up to a week or more of steaming just to get to the hunting grounds. The fleet boats were also designed to operate with the battle fleet (though they never did so in practice), hence both the name and the high surface speed. Now the IXD2 U-boats considerably outranged the fleet boats, but they also had 2 vs. 4 engines, and the engines were smaller and used less fuel.

A lot of the comfort in U.S. boats came from the large size. If your submarine is the size of a destroyer, you can install everything you need and still have room for enough bunks for everyone (at least at the beginning, before crew sizes grew as new equipment was installed). A major habitability factor was air conditioning, which was really there as a dehumidifier. The fact that it made the boat more comfortable for the crew was just a bonus. It was really there to keep the hull from sweating too much and to prevent grounds. There was also a presumption that missions would normally be a couple months long. German boats, operating for the most part much closer to base, for the most part ran much shorter patrols.