The U-Boats were little boats. Of course they were a bit more nimble than the larger American subs. The Type IX had comparable dive times and comparable size to the American submarines. When you're talking much less mass, you need less buoyancy to keep her up on the surface safely, which means less water to take on to get to neutral.
The crew is a much higher percentage of the weight of the submarine, so their positions within the boat influence the pitch angle much more than they would in a larger American boat. One thing you give up with larger size is nimbleness. But you gain the ability to take enough firepower to win a sub war. The Germans needed a lot more subs to have a chance. And they didn't have enough manpower to run the subs even if they had enough of them.
That was a major part of their problem getting the Type XXI to war. Nobody could run them on a war footing and they couldn't spare the crews from active duty to train them.
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