I think there is more than one issue here.
Perhaps your USMC lecturer used a commonly accepted picture of Japanese as short legged and used to sitting in seiza almost from birth. I have just returned from the Netherlands and can accepted another picture, this time of Dutch men, who have uncommonly long legs, in proportion to the rest of the body. Add American 'bulk' and you can assume that the composite picture of a 'westerner' would not fit Japanese prisons, even if both are really a caricature. I spent ten years as a member of a local police support group and we were once shown round a Japanese detention house. The cells are not large, but it is nonsense to suggest that they are too small for 'average' non-Japanese.
The second problem is that of aging. Japan's population is declining and you have the situation of older Japanese living alone. Actually, I live alone and all my immediate neighbors also live alone. They are women and have lost their husbands--in quick succession. I am over 70 and my neighbors are older, so the combined age is over 300 years. The social services are quite good, but you have the prospect of older Japanese--especially women accustomed to looking after husbands and raising children, living alone and having nothing to do.
Best wishes,
Peter Goldsbury,
Forum Administrator,
Hiroshima, Japan