@ goalsiel: one thing, though... the engine is usually the smallest part of the game, all it does is interpret commands, no more than a super-complicated if-then list. Media Files are the real killer. Think about this, each area has, lets say 1 mp3, an mp3 is usually areound 4mb in size, so for 20 areas, that's 40mb. Then each soundbyte is probably 200kb (.2mb) so if there are 1000 bytes, that's another 20mb (and there are probably around 3000 in most of these games. Movie files, like opening/ending and cutscenes, are anywhere from 50mb each to over 100mb each, depending on quality and format.
Then, after that is all the code for the game itself, Databases (database of items, database of characters, characcter lines DB, etc), and the code that strings this all together (which in and of itself is probably close to 30mb, just because of the sheer number of items/lines/etc that interact.
The only reason they're able to keep the filesize reasonable is nothing in the game is high-res, including movies. IF they made these photo-realistic, then teh size woudl skyrocket to several gb per episode!
Most of what you download _IS_ exclusive to each episode. Yes, you could download some basic sound effects like doors and gunshots, and some animatics like Sam and Max themselves, but the vast majority of it is still unique. This is why an updater would be mroe trouble than it's worth - you're only removing 50 mb or less from the download.