According to the latest Japanese government statistics (from 2003), the average Tokyo apartment that is home to a four-person family allows them a measly 36.5 sq. meters to live in. That's just a bit more than a large shipping container.
For comparison's sake: The average size of all the apartments built in Australia in 2002-03 — from studios to palatial pads — was almost four times larger, at 134 sq. meters.
Of course this only confirms what Japan residents already know: Abodes here — often scathingly referred to as "rabbit hutches" — are typically on the very small side of small.