The average is higher but so is the median. A lot of the densely packed slums in US cities were physically destroyed which got rid of a lot of the homes that had very very small space per person, and newly built homes in the ever expanding suburbs after the roads were built out take up much more land and space is at less of a premium than in the cities where the land was very expensive and required building up and new construction standards were significantly higher than they had been.

On the good side this meant that families with a home had more space to spread out in, on the downside the destruction of the slums made it much more expensive to live in the cities with everyone competing for the remaining bigger homes and brought down the urban core populations (Manhattan is still down ~700000 prople from the peak of its population in the early 1900s) and the physically spreading out into the suburbs requires much morr inefficient infrastructure, more environmental impact, etc. Urban people who may have in olden times still been housed in cheap tenement slums now go without and are homeless because there’s a cliff between the streets and the most affordable housing whereas the slums used to make it a more gradual slope.