How do!
Another poser for you to get your brainteeth round.
The core of the 40k setting, like it or not, is Chaos V Imperium (largely Chaos V Everyone, but mostly they're after the Imperium).
The Heresy saw both sides fairly comprehensively shattered. Imperium lost it's cohesion and a colossal chunk of it's military might (both men, materiel, and of course knowledge). Chaos gained part of that might in the form of surviving Traitors, but as we have seen in the modern day background, the cohesion of the Traitor Legions is all but gone.
But from a purely logistical point of view - which side is in worse shape? I'm using quite specific wording here, as I want to ask as neutral a question as I can!
Right, off you go!