It's understandable to be confused about why an old school adventure game is suddenly taking up more than 2GB of space on your hard drive, especially when the Xbox Live Arcade version is only a quarter of that (just over 500MB). LucasArts' Twitter has explained the size discrepancy for The Secret of Monkey Island: Special Edition -- it comes downto compression.
Or, rather, a lack there of on the PC version; it's uncompressed audio and art there. The Xbox Live Arcade version, on the other hand, required compression. G4 has played a little bit of the game on Xbox 360, however, and didn't notice any issues with it.