Even with the previous generation systems, the total size of the game didn't always make a difference between a good game and a bad game. IMO, games like Metroid Prime had graphics that could rival anything on the Xbox and Playstation 2, while being a GameCube game, was limited to a total game size of around 2GB... Roughly half of what the other two consoles could have. It's not so much about the total size, as how efficiently that space was used.

But... The 360 and Wii use standard DVDs or dual layer DVDs, so anywhere from about 4-8GB per disc. The PS3 can use standard DVDs, DVD-DL, and Blu-Ray media. I believe current Blu-Ray media can store up to 40GB/disc.

Won't make much of a difference though. Since the latest generation of consoles are so expensive, and have furthered the push towards more realistic graphics (usually at the expense of gameplay), this has forced most game publishers to go cross system with their games, and release on at least the 360 and PS3. This means they have to focus on the lowest common denominator. So the 360 and PS3 versions of the game will look and act nearly identical when it's all said and done.