16 days ago - Shurenai - Direct link
Steam shows two numbers.
The Download Size
and
The Application Size; As in, the total size of files modified/changed.

The way steam does things, is it downloads the patches, and then it makes a duplicate of all affected files, applies the patches, verifies them, and then replaces the original.

Thing is, Depending on the game you could have a 2mb download that applies to 100gb of files, because it changes 0.1mb in 20 different 5gb databases/glorified zip files (Files that are separate but shipped together as a single file for compression or other reasons)