Hmm - it is strange and hard to "calculate".
That is right that I play Terran race now and attack minor race.
I attacked system 50mld population with 5 x Chillmark-heavy-cruiser and 12 x Security-forces.
Effect: 3 mld killed only...
I'm powerful with science so there are no more advanced race than me. So I thought that the invasion depend from technology level too.
It is strange that the most technology advanced race cannot attack 1-system minor race...
In effect I lose too many resources to fast attack (and no effect!) to expand my empire at critical time (no solid borders between races).
Result: no battles, no planet re-capturing, no tactical movement but technology race to better units or military forces only, which allow spend not-too-much resources to build units.
What now?
To capture this system with one attack I should use 3x more forces... 50mld/5 = 10u x 3 = 30 units of security forces, and 15 transporters to keep them.
And it is only minor race with one system...
It is fine if I kill 30-50% population with buildings... but not less than 10% only.