Again with the " not nearly big enough". Several years ago I did a little research on the developed land in the USA. Based upon the information I was able to find, all of the land area covered by the boundaries of all the cities and towns, and the areas within all the highway and road right-of-ways, was 8% of the total area of the 3.8 million square miles of the USA. In few countries it is more, but in many other countries, it is less.
No one I know of, except you, envisions splitting the population of Earth onto other planets. The population of Earth will no doubt continue to grown and expand within its physical and economic limits. Those limits are already slowing the rate of population growth. Colonies established upon other planets, such as Mars, will also grow within their own physical and economic limits. Due to the extreme conditions on Mars, those limits will dramatically affect the rate of growth, but that is natural and will be an integral part of the economic growth and expansion.
All that will be true for any off-Earth colony or settlement, be it on a Planet, a Dyson Sphere, or a Larry Niven type Ring World. Keep in mind that every off-Earth structure, especially a Dyson Sphere, or Ring World, would require extremely enormous amounts of construction material. In and of itself, creating that, transporting it, assembling it, and outfitting it to be a viable place to live, would limit how fast it could be done. All the factors involved in off-Earth habitation will always be automatically self limiting.
Who in the hell was talking Dyson's Sphere or Niven's Ringworld, the worst things possible for a civilization to engage in building! Effectively stone tombs the advanced monolithic death wishes and super monolith coffins of an advanced species dreaming of nothing more or less than an energy-less suicidal Utopian Communism.
Otherwise, you are thinking and dealing in things strictly 2-dimensionally horizontally lateral rather than multi-dimensionally expansively horizontal and vertical and everything in between. You certainly know nothing about entropy in flatly closed systems and throughout recorded human history concerning civilizations everywhere and everywhen situate. Nor anything concerning the physic of an Earthly black hole you are dreaming of simply extending to more than one world. Nor do you know any difference between real systematic growth and closed systemic bubble inflation spinning wheels ever faster on a treadmill going nowhere, Nor tyrannies (particularly Orwellian). Nor anarchies (again, particularly Orwellian). Nor pestilence, war, famine, and death . . . nor Ice Age.
Species, and human civilizations and other groupings, don't slow in population growth, they grow into extinctions, as has been worrying many nations now whose declines in societies and economies, have followed right with declines in human fertility, virility, biological physical and mental, and [emotional caring], energies, civilities, population willingness to populate, and more. There is nothing on Mars to draw any attention of 99% of humans. There is plenty to draw them to '2001: A Space Odyssey' and 'Voyager' Stations, Stanford Tori, O'Neill Colonies, spaceships and shipping, and so on. They aren't stupid, they are spatial seers and thinkers (even the mediocrity of the human species can think spatially). They, the 99%, can see themselves there in those in-space stations, those in-space colonies, those in-space ships, They, the 99%, descendants of those who covered the Earth including the islands of the mid Pacific in nothing more than large barge canoes, and so on, cannot see themselves as any part of the limited and limiting colonization of Mars, the Moon, or caves in other rocks of the Solar System. There are no possibilities of frontier vastness, frontier energies, and thus frontier opportunity, creativity and prosperity out there and down here, there on Mars or in caves in the Moon and other rocks for them. Colonizing Mars and/or the Moon are very low energy -- and everything else --propositions for the 99% vast majority of mankind.
With in-space occupation beginning and ongoing, the 99% will begin to feel the exhaust to New Frontiers from the Old World. The most of the 99% will begin to feel almost immediate synergy in growing energy exchanges (money is a token of energy / life / frontier, a token of momentum). Colonizing Mars and the Moon, and any rocks directly, offers nothing for the 99% but your promises of possible maybes hundreds to thousands of years down the road. Pursuit of [in-space] promises immediacy of momentum growth and expansion, energy growth and expansion, and possibilities expansion and growth, for the vast majority of humanity and life.
You say the Earth is big enough, and Mars is big enough. I'm sorry but you have no clue, not the slightest clue, about an entropic closed system. Nor about the historical mind of a natural frontier -- otherwise naturally declinist -- species.
Now, building and transporting to Mars orbit, spin-gravity space stations and a Stanford Torus cloud-city-state colony or two would really begin the permanent colonization of Mars and the outer planetary systems beyond Mars. Still a side issue, but a stage on the way in growing out system in cloud-city-state in-space colonies colonizing the orbits of all the planets and bigger moons going away in the frontiers of the outer system.