Your armor must have out teched their weapons because you sure suffered light super heavy losses. Sure seems like big vehicles are excellent for assaulting planets.
The best armour on the NPR ships I captured was laminate composite armour. I had only recently achieved biphase carbide armour so given the time ultra heavy vehicles take to produce even having maxed out ground force construction rate most of my forces would be compressed carbon armour and earlier types of armour, hence its principally the first generation units being knocked out. Thus I was not that far ahead of the NPR in armour tech and could not afford to leave any units on the sidelines regardless of their armour!
I have only 32 troop transports with a 50,000 ton capacity and 16 with a 55,000 ton capacity, so I prefer ultra heavy vehicles. Transport capacity is on the tight side. My new standard ultra heavy being armed with 1 x SHAV and 3 x HCAP. My formation size is limited to 10,000 or under. It is helpful that the NPRs don't appear to build more that medium vehicles, but they do build a shed load of them (and infantry).
I am getting tired of the asteroid and cometary forts the NPRs build, usually protecting nothing with up to 200,000 tons of ground forces plus several thousand tons of STOs visible.