That's kinda strange. Obviously, if you want to burn it to a disk, it's going to take the full size of 4.5GB (to my knowledge, there's no way to store multiple games on one). Running the game from an USB-drive, you should be able to use only the small size (more or less the size of the compressed ISO).
I advice using wii backup manager. No matter your file system (are you using WBFS, FAT32 or NTFS?), it should be able to shrink it to the 'real' size.
In this post a guy says there's a pattern for the information that is garbage... Based on this, I tried to replace everything from offset 010228000 to 108820000 with 0's but I had no sucess... Anyone knows the right thing to do?