Is this a reference to American obesity or something else..?

No, more that American hotels are a lot larger than European ones (I'm from the UK) and I assumed (with no real evidence) that Americans carry more luggage.

Although it is true that Japan is a bit of a struggle if you are overweight or especially tall.

One of the best room I ever had was in Norway. It was at most 3 times as large as the queensized bed, had at least 5 different lighting options and a fantastic black-out curtain (this was needed above the Arctic circle) and tons of small storage places and hooks - and this includes a tiny bathroom. Everything you need, and super cozy.

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.