My violin isn't Japanese buuut i got my old 4/4 violin out to compare... my new one does look much smaller! It's nearly the same length but is a lot thinner - though my old 4/4 is a big Strad model.
She's very old and is German
I think the measurements are possibly similar to full size but everythings's been made a bit narrower and smaller? gah, i can't explain what i mean

Edit: i just realised that you meant there are 7/8 violins IN japan, not JAPANESE 7/8 violins haha oh silly me