Not sure how scientific this theory is, but I assume that a smaller boat would flex less; i.e. it's easy to bend a very long steel bar with your bare hands but a short piece of one is going to be tricky as you wouldn't have the leverage. So I assume a shorter boat would have better torsional strength, which might be a factor when the water pressure begins to push down on things at diferent positions on the hull.
With US boats being initially designed for fleet actions in support of surface ships - as opposed to German ones designed to act independently from the outset - they were considerably longer, and presumably would flex more.
Ever since WW1, Germany was noted for being well in advance of other countries in techniques for metal fabrication (notably welding). This is remarked upon in many allied documents with regard to captured WW1 German equipment, so maybe this is also a factor.
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