Well, it's up to the developer...
MotorStorm for example uses less than 1MB but when you download the update it goes to 22 MB.
To be honest, i don't mind using 22 MB on a better game.
And if you need more space, you can just upgrade the HDD.
Just as a question, didn't a lot of PS2 fans put down Microsoft's entry into the console market claiming that Microsoft would lead to people needing to upgrade their systems and download patches for games?
I remember seeing ALOT of Sony people complain even upto CoD2 and Oblivion on 360, that MS was forcing online so much that companies would release half-cocked games and patch them to death once the games were out ala-PC. Now PS3s are doing it, and I would think Wii will at some point too.