Perhaps the galaxy is something like this, in terms of a political map? Just an inaccurate scetch.
- 400,000,000,000 stars in the Milky Way Galaxy
Mankind:
- 100,000,000,000 stars under Imperium's borders
- 1,000,000 worlds inhabited by Mankind
- 50,000,000,000 star systems explored/mapped, numerous listening posts, automated outposts
Eldar:
- 12-40 craftworlds under Eldar control
- 1,000 Exodite worlds inhabited by Eldar Exodites
- 100 colony worlds created by Craftworld Eldar
Dark Eldar:
- 1 megastructure under Dark Eldar control
Orks:
- 20,000,000,000 stars under Ork Empires' borders
- 400,000 worlds inhabited by Orkoids
Necrons:
- 80,000,000 stars under Necron dynasties' borders
- 10,000 tomb worlds inhabited by dormant Necrons
Chaos:
- 50,000 inhabited, largely tangible or corporeal worlds, in the Eye of Terror
- 15,000 inhabited, largely tangible or corporeal worlds, in the Malestrom
- 50 space hulks used as bases by warbands
Just a completely speculative bit of political/demographic mapping; probably the Imperium is nowhere near this neat, and the numbers are totally off, being a fantasy conjured out of thin air. The Imperium faces more enemies because of it's size, bordering everyone, but each Ork Empire, plus Chaos region, is a defencible rival nation state, and Eldar Craftworlds can project huge military power due to the Webway, containing billions of souls each, so serve as an umbrella over the Exodite Worlds. Total war isn't prevented so much because the Imperium is unbeatable, or overwhealming, but might lose another segmentum to rivals; but rather because each major Ork and Chaos Empire can legitimately hold it's own against the Imperium. Sieges are hard, territory costly to gain for everyone; Ork Warbosses are not dumb dey "systamatakilly" patrol their borders with fleets of Kill Kroozas, Tomb Worlds are dug in, and the Eye of Terror is fortified with layers of Imperium-grade planetary defence.