I don't use Launchpad heavily, but the larger icons are more practical. When you group programs together, you see the first nine within the group. Previously, the sizes of the icons were too small to be able to easily identify what the programs were (unless you were extremely familiar with the program icon, perhaps). With that size, it would have been better if they allowed you to use a folder icon, instead of trying to represent the programs that were grouped together. With these larger icons, I find it much easier to pick out what programs are within the group.
For what it's worth, I'm in a 15" Macbook Pro. Maybe the problem of seeing programs within a group didn't exist on systems with larger screens.