Dynamic VHD size increases drasticakky after modifying BlockSize to 256 MB from Default 32 MB.

Installing Windows 2012 R2 under Hyper-V virtual machines, the disk size in the end is around 9 GB but in case the BlockSize is set to :268435456. It goes above 11 GB.

Then doing the same operations on a differencing disk the size goes to 4 GB in case the block size is 2 MB but 14 GB when the BlockSize is 256 MB.

Any ideas why?

yup