Just played some more and I did actually take losses of up to 20% on some invasions, though the battles were generally of the 100 vs 6000 sort (with me losing 0 to 20 and them all dying).
However the tech factor that I mentioned above seems to be the big thing, but I don't know how that is calculated. In my case though it was something like ~30k vs 3k.
So I guess the real problem is how is that rating determined and how does it affect the battle outcome? I would imagine in Jack's case he simply has a very high tech rating when he invades, and thus mever takes losses....