[Nyororin]

There is a fair difference. Back before vanity sizing changed US sizes (10+ years ago the size standards were different), a US M was a Japanese L. Now it seems that a US M is a Japanese LL or even 3L.
Even measurement based clothes have been hit by the vanity thing. When pants in Japan say they have a waist measurement of so many cm, they have a waist measurement of so many cm. Apparently US brands give somewhere between 3 and 5 extra inches over the listed size. US shoe sizes also tend to be what would be called a 3EEE in Japan - designed for wider thicker feet... So what is a size 8 in the US would be more like a size 27 3EEE (virtually impossible to find in normal stores.)
My husband can wear size 8 US shoes quite comfortably, but we have to hunt up and down for 27.5 3EEE shoes to fit him.
Bra sizes are on a different scale altogether and are designed for a different shape of chest, in my experience.

Regardless though, I find it hard to imagine that someone couldn`t find clothes small enough in Japan.