Probably because they pack a ton of files into a handful of compressed archives, which means the full archive needs to redownload when a single file is changed. Does Steam not have a delta patching system to handle this? They already compress downloads so it's not like a delta patch framework would cost extra CPU in comparison, and the bandwidth savings would be immense. Seems like low hanging fruit to me?

If it makes you feel any better, patch 4 actually freed up 20 gigabytes of drive space for me once it finished installing.

This must be your first time receiving an update. I'm surprised it's so small.