There are many programs out there that scrub games...
As far as I know, cgz will only work with dolphin... not a very good solution, but if you only ever plan on using dolphin not a terrible one.
First I think you have to understand what Scrubbing actually does. In Wii games there are basically 2 components... The actual game data, and the filler. The filler is just random data to pad out the games to the full size of a DVD (4.7gb for single layer, 8.5 for dual layer). Most games come no where near filling the DVD, and some take hardly any space at all (wii music, for example, is only a few hundred MB in size) but all that random data compresses terribly.
That's where scrubbing comes in, scrubbing removes all of those random bits of useless data with 00's or FF's... Both of which compress incomprehensibly well. The actual ISO itself is still the same size, but when you use any form of compression on top of it, it shrinks the game down nicely.
TL

R Scrubbing doesn't make games smaller, it makes them easier to compress. Some form of compression or conversion is necessary to get the game size down. I again recommend just using NTFS compression since you don't have to convert anything ever and can easily move it to different locations/devices as needed.