My thoughts is that nothing about Warhammer makes sense, and it is fine that way.
Seriously though, it`s either one Space Marine is literally better than a million IGs, or there are vastly more SMs, or SMs are basicly amount to nothing in the large picture.
Logically, there should be billions of Space Marines. A chapter for each world at an absolute minimum. I mean, just 1000 supersoldiers for a whole world is peanuts.
But logic was devoured by Tzeentch during the War in Heaven, so yeah...
Space Marines don't amount to much in the big picture, the Astartes as a military organisation are no where near the power levels of the Guard or Navy.
Individually each Marine is one of the best warriors humanity can create, and the Chapters aren't hampered by Imperial bureaucracy any where near as much as the other Imperial organisations (exc. the Inquisition) which allows them the full freedom to play to their strengths.
But if the Astartes went up against the Guard and Navy, head on, they wouldn't even last as long as a snowball in hell.