>Living in larger family units is actually very efficient.
Maybe so, but it's not a sign of excess wealth. Japanese young people live with their parents longer than was customary thirty years ago, so it's not a question of culture. They simply can't afford to move out.
All that means is that Japanese young people are getting married later than was customary 30 years ago. For the eldest son, it's still expected in some families that he will never move out, even after getting married (my brother-in-law was roped into this, after having moved away to Kanagawa. He gave up his job and moved back into his parents home because his father more-or-less ordered him to). It's cultural as much as economic.
Which is the chicken and which is the egg? Are they moving out later because they're getting married later, or are they getting married later because they're moving out later?
Living in larger family units is actually very efficient. The way we do things here in the U.S. is actually shockingly inefficient. Parents buy big houses in the suburbs when they have kids, and don't down size when the kids leave. So you have an older couple using one floor of an oversized house and the kid maintaining a separate household. Then when the kid has kids, they buy another big house in the suburbs, then pay someone thousands of dollars a month to take care of said kids after school. Meanwhile the grandparents sit around their big house bored and lonely.
There are a lot of weird cultural taboos in the U.S. that create this state of affairs, but the end result is vast inefficiency in resource usage compared to societies where bigger households are the norm.