system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] How can Space Marines be a significant army if they are so hopelessly outnumbered?

I can accept that individual Space Marines are nearly indestructible engines of slaughter and mayhem. But the fact remains that they are  armed with melee weapons and line-of-sight firearms while traveling at the speed of feet in quantities of 1000 or less. In the time it takes a Chapter to kill 10,000 Orks/Chaos worshippers/Necrons/etc, 100,000+ other enemies can sweep across the rest of the planet unmolested.
gpt
Space Marines aren't an army, just as US Marines aren't an army. They are a rapid-response initial landing force. Their job is not to scour the planet, their job is to set up a forward base of operations so that the real army - The Imperial Guard - can some in and clean the planet off.