Most of the space in cities in the USA is taken up by unnecessary car infrastructure. If it was eliminated, cities would be smaller, there would be no need to have a car, commutes would be shorter, and people would have more space. Because of the USA's economic system, people are incentivized to break each other's legs in order to sell wheelchairs.
We haven't colonized much of the underground. If we built down and up instead of to the side, the USA wouldn't be so much of a dystopic hellscape. It's the lack of land management, terrible zoning laws, and lack of a planned economy that allows this to happen.
If we colonized Mars, it would be a tyrannical, corporate nightmare where people work fake jobs in order to buy products and services they don't need. Until we find a way to allow people to work for good instead of just getting paid, make companies try to make people less dependent on their products instead of more, and eliminating wealth inequality, there's no point in thinking about colonizing Mars. With the way things are now, it would be a citadel of crime and corruption.