It's using diff files to patch your pre-downloaded files. That tends to reduce the size of patches compared to the files it's going to patch.
Thanks for the quick answer, I never would have guessed that; thanks again!
Yeah, I should have mentioned I'm on PC; I'll edit my post!
On top of that, the patch is unlikely to contain new models, textures or music, which are the things that take up bulk of the size of a game. Game code and map scripts are not that bulky.