When lots of people have lots of babies on Earth, we just sprawl out.
Having lots of people with lots of babies in a floating tin can is not a problem we can easily solve with our current level of technology.
Within my lifetime, I have much more faith that we'll be able to construct habitats out of dirt on planets with an atmosphere than modules out of asteroids in the vacuum of space, and that's the appeal of planets in general, and Mars in particular.
Realistically, "Colonies" will not be colonies for a long time, just the equivalent of barracks; people will live there for a while, but their main lives will not be in space.
Having lots of people with lots of babies in a floating tin can is not a problem we can easily solve with our current level of technology.
Within my lifetime, I have much more faith that we'll be able to construct habitats out of dirt on planets with an atmosphere than modules out of asteroids in the vacuum of space, and that's the appeal of planets in general, and Mars in particular.