VHD file size

Hello,

I have been using Hyper-V 2012 but having some issues with VHD file sizes. For many VPSs, the VHD files are sized about double than their actual sizes inside VPS. Even if I delete half of the data inside VPS, the change doesn't reflect in the VHD file size. How can I reduce the size of VHD?

Thanks,
Deepak