Windows backup takes more space then used space on disk

Hi everyone,

I have an issue with Windows server backup on Hyper-V role.

I installed Hyper-V role on WS20012R2, and installed virtual machine on hyper-v with two fixed disks.

(VM is Exchange server 2010). First hdd is for system (80GB), and second hdd is for Exchange DB&LOGS (300GB).

Used space on system disk is 40.5GB, and on second disk 50.4. So, backup VM should take about 90GB.

I try to backup virtual machine named 'ExchangeSrv', but my backup takes 130GB, but when I run full backup from Exchange Server, my backup then takes 91GB.

Do you have any opinion about this issue, or some suggestion ?