Oh, you want to make chd files? It sounded like you wanted to compress something that is already compressed.
With the release of .74, a buddy of mine found a redundany in the way CHDs are handled. It seems that a lot of duplicate bytes are included in the existing CHD files.
So, with .75, hdcomp.exe will recompress to this new standard. All CHDs that are recompressed are backwards compatible with older versions of MAME, but take up less space on the hard drive.
Since uncompressing/recompressing using hdcomp.exe takes up a bit of drive space, there are two utitities that were written to just output a new CHD without having to recompress them first.
One is ReCHD. Written completely in Java. Nifty, but you have to have Java installed.
The other is SmallCHD. Does the same thing as ReCHD, except you don't need Java.
The existing CHDs don't get squashed too much (I think Area 51/Maxforce gets quite squashed, but the rest don't too much) but the new Beatmania CHDs are far smaller, saving over 1.5 G overall.