system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] What is the size difference of the Imperium post heresy as compared to pre heresy?
gpt
I believe the actual size of the Imperium remained the same, more or less. While some planets may have turned traitor, they would have been reclaimed once Horus was dealt with and brought back under Imperium rule. That being said, the Great Crusade wasn't finished when the Heresy started, so many parts of space were still undiscovered. Given the 10,000 years between settings, it's actaully possible that some of this unexplored space would have been settled.

In summary, the Imperium was likely to have grown after the Horus Heresy, though their gains would be small and barely worth noting.