Will these domed planets ever grow larger?

[raynor]

You can convert the domed planets to undomed planets by fully replacing the colonists with ones that breathe that atmosphere. You can accomplish this by either conquering an empire whose colonists breathe that atmosphere. Or... you can establish a diplomatic dialogue with that race and try to trade for one of their colony ships. Colony ships have 4 million colonists. That would be enough to initially convert four domed colonies to undomed.