They are "small machine worlds" because they are small planets. There are no other sizes of machine worlds.
You should be able to change the atmosphere of an irradiated planet, but you can't get rid of the "radiation" because that is a planet size, and has nothing to do with the "atmosphere." Also, non-domed colonies have fewer slots than domed ones do on the irradiated worlds, so leaving the atmosphere alone is often better than changing it.
[ March 24, 2003, 18:45: Message edited by: Imperator Fyron ]