The reason is probably pretty trivial: older Defraggler just by chance happens to modify the drive in such a way that the Shadow Volume subsystem doesn't use up too much space for the system restore point. It's a sheer coincidence. Newer Defraggler does the modifications in such a way that Windows uses up significantly more room for the restore points, and removes the old ones much quicker. What you're experiencing is a designed-in "feature" of Windows with unintended consequences. There is nothing, zero, NADA, that can be "fixed" on Defraggler's side. If you don't like it, you can completely