C: Drive shows less size than original size assigned

Hi, I'am new in Hyper-V. One of my client using Hyper-v host in windows server 2008 and having issue on their internal C:/ drive size assigned.

The C:/ drive shows 50GB size and 7GB free space. When I check in Disk management, the storage assigned to C:/ drive which is "Disk 0" is 450GB. How to make the C:/ drive have the original assigned drive size? 

Hope can get some help here.

Thank you very much.