Japan Travel Fixes
Why are Japanese hotel rooms and beds smaller than I expected?
Land is expensive, especially in city centers, so business hotels are built with narrower rooms and beds than you might be used to elsewhere. Checking the bed size listed on the booking site (single, semi-double, etc.) beforehand can save you a surprise.
Groups of three or more often have better luck with a Japanese-style room or a vacation rental than trying to fit into a twin room with an extra bed.
Upscale hotels and resorts tend to be closer to international room sizes, so upgrading the hotel grade is one option if space matters to you.