When you are browsing the Game marketplace for new Arcade titles to play, you're looking at a collection of titles that weigh in at under 50 MB in size, with few exceptions.
The first that comes to mind, Portal: Still Alive, is a game separated from its mother, The Orange Box, and raised to become a charming young game with a few more pleasant additions. This title clocks in at 629 MB.
With the size of Portal: Still Alive not being streamlined to fit the maximum capacity, you can get an idea of what you're looking at with a 600-or-so MB game. (Please keep in mind that all games are made differently, with different engines behind them, and the file size doesn't mean quality.)
With the total capacity for all XBLA games being increased to a whopping 2 GB, I wonder what else is in store?