Meu palpite é que isso pode se basear no fato de suas unidades serem ou não volumes fixos ou dinâmicos. Verifique essas configurações e leia as diferenças aqui: link
Em particular
Dynamic VHD Benefits
Quick allocation of space. Since a new dynamic VHD contains only header and footer information, it is extremely small and can be created almost immediately.
Minimal space usage for VM-level backups.
Backup utilities that capture a VM operate at the VHD-level and back it up in its entirety no matter what. The smaller the VHD, the smaller (and faster) the backup.
Over-commit of hard drive resources. You can create 20 virtual machines with 40GB boot VHDs on a hard drive array that only has 500GB of free space.
Read more: http://www.altaro.com/hyper-v/hyper-v-guest-design-fixed-vs-dynamic-vhd/#ixzz3NpQFJQPy