As I think you've guessed, the trick is to get things as small as you can at the time of imaging. When you run VMware Converter and it asks about which drives to image, there is a little drop-down next to each drive letter where you can select to keep it the current size or use the smallest amount of space (IE only the data). That may go a long way towards reducing the size. The other option you have is to make the image "growable" or fixed size...if you use growable, then move the image to your Mac, you can boot the windows image and remove anything you don't need...the disk will shrink accordingly.

As a matter of practice (notice I did not say "best practice" b/c I'm not totally convinced it has much real value) I do keep my data and OS on separate partitions (with a 3rd partition for swap files). I've found that does make things like imaging and backing up a tad bit easier. But it sounds like its too late to go that direction now...