I'm sure some more VM-savvy people will chime in here, but it sounds as though the delta file is doing what it is supposed to.
As far as I know, snapshots work via copy-on-write, where you start out with the original image (that's your vdisk) and an empty file (that's the delta file). Every time anything is changed at all, that change is made on the delta disk. The original disk isn't touched, that way if you ever need to revert to a snapshot, the delta file is thrown away and everything is read from the original vdisk.
Over time, this leads to the side effect of the delta file growing massively as things are changed, added, and removed. As I understand it, if you add a 10MB file, the delta file grows by 10MB. Remove that file, and it grows by another 10MB, because there is a 10MB difference. I could be wrong, and it might actually shrink by 10MB, but I don't think so. (Please someone correct me).
If you consolidate your changes into your snapshot, you'll be back to the original 50GB disk image by itself.
Of course, I could be horribly wrong and mistaken, in which case I'll be voted down, and you should listen to whoever comes in and knows more.