Americans are very fond of wide open spaces and square footage. Imagine being from Colorado, Wyoming, or Montana...or even California...and staying in a Japanese salaryman hotel. It's going to be claustrophobic for some people not used to the experience.
The hotel advice is if you're paying normal hotel prices you're going to get a room that's not absurd.
It's more that Japan (and many other places) have a product class at a pricing point that Americans do not so Americans think they've gotten a great deal but instead it's a product they're not expecting and not familiar with.
Maybe you want that, just know what you're buying.
There are hotels in Europe with rooms as small as cruise ship ones (I stayed at one like this called the cab-inn in Aarhus). Mountain lodges in national parks tend to have small rooms also, especially if built during the 19th century.
