I visited my friend and asked to use their restroom and they had one of the smallest I've seen so far.
Here it is
Before western style toilets came to Japan generally Japanese would squat over a hole in the restroom floor. I'm not sure of the details how the stuff got from under the house to somewhere else. As they modernized they started making them more like toilets were you could actually flush them. There are still tons like this
This one is from my school. The 3rd floor is this style, the 2nd floor is western style.
There are advantages to this style. One being that no part of your body has to touch any part of the toilet. If you have explosive diarrhea I suppose...well I guess I don't want to imagine that 🤢
Anyway, if you look at the first picture, it is a toilet in the style of the second picture coverted to western buy building a seat over the old toilet.