Steam's reported download sizes are usually attributed to how large the total of the files affected by the update are because it uses alpha patching. Been down this path already once or twice this year.
Valve guy 1: I'm bored
Valve guy 2: Me too.
VG1: I know, lets put out another fake HL2 update!
VG2: Yeah, what should we put in it?
VG1: I dunno, just make it very large and have it display Korean text.
VG2: hahah, sounds good.