Hi FroBodine, I'm afraid I don't have a good answer for you. Steam's patching process is a mystery to me!

This has to do with the way the game is packaged. I'm not exactly sure on the the details but as far as i know, because the game is mostly a single heavy file steam has to verify this file everytime anything changes. Does not matter if it is a 2mb or 200mb update, verify the entire file it is.

Path of exile used to have the same issue, not sure if they solved it:
https://steamhost.cn/steamcommunity_com/app/238960/discussions/0/2590022385673291958/

Originally posted by The_Driver:

Steam doesn't patch the game, it replaces the changed files with new ones. So while it updates the game it doesn't actually patch. PoE is mostly one container file, which as the time of the last update to this post is about 26 GB in size.

This means that steam will:
- Preallocate a new file, this writes around 26 GB to disk
- Fill the new file with data from the old file and the downloaded chunks, so reading and writing 26 GB each for patches that reuse a significant amount of the file (most patches)
- Delete the old file and move the new one

Check The_Driver's response for more details.