The trick to making biospheres small is to increase their topological genus to set lower bounds on the number of feedback loops and reduce internal interaction surface, e.g. cycle through temperature/light/moisture conditions and let inhabitants timeshare through hibernation,
If you attempt to survive in very very inhospitable environment (e.g. Mars), then you get better at surviving in a somewhat inhospitable environment (e.g. Earth). A happy by-product.
This is why I like the Bratton / Strelka framing of the problem as “terraforming”, ie dealing with the technical and social problems of planet-scale systems
https://theterraforming.strelka.com