Something odd I've noticed about some of the GOG files I've downloaded is that the size of the downloads exceeds the size of the actual game after it's been installed. It got a little jarring when I downloaded Amerzone and noticed that the size of the .bin files needed to install it were more than double the size of the complete game. How does this apparent inversion of file compression work?

Evil magic? If i wasn't for the fact that its doubled, I suspect it could be a space on disc thing, if the drive is fragmented it will drop bits of data all over and might just edge the file size up a bit more.
With double the size I'm thinking maybe its one of those games that had multiple discs which had largely the same content. The infinity engine games were notorious for that, planescape torment had 4cds but the amount of unique data was something just over a gig but they had the duplicates on the discs to reduce swapping

I'm hoping this was a thoughtless typo.

heh yeah I noticed that myself but forgot to comment on it

I don't have the game, but Amerzone is multilingual. You can choose different languages when installing the game.
As reference you can take a look into the multilingual games thread:
http://www.gog.com/en/forum/general/list_of_multilingual_games/_/1

Aren't the .bin files just disk images? If so, there is no compression, its a 1:1 match for the original disk. Not everything that is on a disk necessarily gets copied to the drive as part of the install, so it really wouldn't surprise me to see a minor difference in sizes.

Exactly, the voice files for five different languages take up quite a bit of space in the installer, and only one of them is actually installed.

Also some games are disk, CD or DVD versions of said games so size may wary quite a bit from individuals retail version and as pointed above if you own a single language version, then obviously multiligual version is larger than yours (depengin on how much voice acting game has, as text usually does not take that much space).

Whoops.
*Finds dictionary*
*Looks up "download"*
*Looks up "upload"*
...Yeah. I should probably edit that.