Why treaty tonnage favoured small U-class submarines

Britain, Japan, and the United States are described as having equal submarine tonnage limits under the London treaty. A U-class boat of roughly 550 standard tons consumed less of that allowance than a submarine of around 1,000 tons. Building small boats could therefore increase the number available without a matching increase in total tonnage. The number of submarines matters to both the navy operating them and the opponent trying to counter them.