This is something I've visually observed all over Australia. The walkable areas are noticeably fitter. I don't think all of those people just happen to care about fitness more, they just spend more time moving to get between places rather than sitting all day.
The original reason for low population density was agricultural work, which - even in the early days of mechanisation - was plenty physical.
Living in exurbia to work and consume like a city dweller is a new kind of stupid.