Also worth noting, armour tech is based on your highest defensive tech tree - I think just armour and shield types, but others might work. (Likewise, GU weapons are based on space weapon techs, so you can get a better multiplier when you unlock a new laser calibre or missile damage multiplier).
The way you increase defensibility on your ground units is to build heavier, and build more of them. It's not represented in as much detail as ships.