Very large files after converting VHD to VHDX

Hi,

On a Windows Server 2012 R2 I'm using the GUI to convert my disks from VHD to VHDX.
All goes well but my 8 GB dynamic VHD file gets about 60 GB after conversion to VHDX!!

I've also tried PowerShell with or without the -Dynamic parameter.

I have a couple of VHD files that do get a couple of GB larger then the original VHD file.

I've read articles on the net where you get around 145 MB extra on the VHDX but mine is getting huge in difference.

Any ideas why this is?

Kind regards,
Bhavesh