Good point. Perhaps they should have no population space at all, just some cargo space and the ability to resupply and launch fighters.
Even better:
<Wild idea>
Treat each asteroid field like a space station or group of space stations - You don't need to colonise it, you just have to get a construction ship there and take advantage of a free "hull" with natural cloaking, really low maintenance and space for scanners, fighter bays, cargo space, weapons etc.
In combat each "hull" (ie each rock) would be treated exactly like a space station. It would have a special graphic and it could be boarded or destroyed just like a space station. When destroyed the rock becomes available for someone else to build on.
Tiny asteroid field - approx 100KT hull
Small asteroid field - approx 200KT hull.
Medium asteroid field - 1 100KT hull + 1 200KT hull
... and so on.
When I say "approx", why not put in a random modifier? One tiny asteroid field has 109KT space, the one in the next sector only 98KT. I don't know i there are any components <5 KT but it would b a good incentive to mod some
</Wild idea>
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There is an exception to every rule. Including this one.