See that in a small ship universe, any invasion would need a lot of transport ships, as the ground troops will still be the same (according to population/TL/government, etc.).
So the troops needed for nearly any misión will be the same than in a large ship universe, but the tropos carried by each transport ship will be fewer.
I was going to try to make sure there were no really high pop worlds (I'm going to generate new subsectors rather than use old ones), with a Pop of 8 reduced by 1 and a Pop of 9+ reduced to 8. Still, the idea of most empires having one world that is invasion-proof does create an interesting dynamic as all the other worlds are routinely fought over.
Oh, and I have GURPS Traveller: Ground Forces, which does go into some detail on invading worlds of various classifications.