I don't think at this point, Parallels has support for compacting OS X disk images. Perhaps in the next paid release. I would not hold my breath for them to add any significant functionality to this release, I've been purchasing Parallels since v5 and their release cycle seems to be annual. Once you buy a version, you can expect them to fix some bugs in the version, but for the most part any added functionality you will have to wait for the next release, and buy it. It's why many of us are frustrated with Parallels.
If you are using a OS X guest, you may also notice that the suspend functionality does not work. If you suspend a guest, it will not not resume, instead it reboots and can cause disk errors in the process (it's a hard system crash basically). Despite coming out with multiple updates since this problem was known, Parallels has not even added a warning message or prompt on suspend. It's pretty frustrating. Perhaps when v9 comes out we can all spend another $70 and that will work properly.
You may also notice that file copy performance over the virtual network can be horribly slow, even between the guest and the host (ie: 100% software, no actual network involved). Perhaps this will be fixed in v9 and we can benefit from that after dropping another $70.
Rob
I try every time there is an update, hoping it will work, but it's never worked as long as I've tried on my setup... ah well.