When updating a game on Steam, there's 2 metrics per game that the client is keeping track of: how much was downloaded and how much was written to disk. I know compression exists and there are some edges cases like <4KB files taking up 4KB on disk (or whatever your sector size is) so it's rare that these 2 numbers are exactly the same. But sometimes these numbers are EXTREMELY different. I'm currently updating Halo Infinite and this is what I see in the client:
Unless the game contains GIGS of redundant data that compresses incredibly well, I'm not sure how a ~600MB download could result in ~35GB of modifications to disk. I don't think Steam is doing anything malicious, but I'm very curious about what's happening here. I see this as a learning opportunity. My wildest guess is that maybe a small utility is downloaded that when run will modify large pieces of data like modifying 3D model geometry for whatever reason. It's a complete shot in the dark that's probably wrong.
I'm familiar with a lot of the optimizations that other software distribution platforms use. There may be multiple features working together to bring the download size as low as possible. Compression, only download what changes, only download patches for large files, but in my mind these don't add up to the 70 fold increase between what's downloaded and what's modified. I've never published a game on Steam before so I don't know what options developers have available to them. Is there any deployment documentation that could explain this?