Slower-than-light biosphere arks start out large but become smaller with tech level, some of the most advanced interstellar ships look like bottle gardens that just happen to travel at 0.1c
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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,