Dmm, Perhaps adding production penalties for colonies with populations below 100M, and more severely limiting the populations of domed colonies would solve the problem? If a domed colony could only hold 1/10 of an outdoor colony, instead of 1/5, a tiny world could only support 10M people. Any reasonable scheme of penalties would be harsh at a pop level of 10M if it took 100M to be 100%. Also, adding a conditions level of Deadly, that only occurs on worlds without your native atmosphere type should help some.
The 1/10 probably isn't balanced right, but the idea itself should be sound.
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[This message has been edited by Nyx (edited 03 January 2001).]