Can't restore a windows disk image to appropriately-sized partition

[Ravenous]

Here's what I'm trying to do: I have a macbook (Generation 2,1) with OS X 6.3 with a new 500gb hard drive. Before upgrading my hard drive, I saved the my bootable windows (NTFS) partition on an external drive (as a partition) and as a disk image, which is 16.3gb. Now I've made a blank partition 20gb in size (just to make sure) and tried to restore either from the external drive or from the disk image (the result is the same). I get the error message that there is not enough space on the destination. Then when I check back, the partition I was trying to restore to has now shrunk to 16.3gb! Is there a way I can get the restore function to work properly?