Even at a planet scale, 1k troops are not enough. Sure, each is worth 100 men or whatever, but that makes them always spread too thin. In a planetary invasion, which orks and 'nids often do, if they can't trick the enemy into chokepoints, they're fucked big time.

The Armageddon war had, what, 20 chapters deploying marines there? And that's with massive support from the local IG regiment, plus space navy