After installing the win8.1, the hard drive capacity is very small what's going on

The reason for this problem is probably that your friend did not format the C disk until you installed the system, but instead installed the system directly, which would result in two system files, and the C disk capacity would naturally become smaller. If you have a Windows (old) folder in your C disk that is the case, it is recommended that you do not delete it directly. Follow these steps:

First, go to the computer (this computer), then click on the system disk (usually C disk), and then select "Properties",

Second, we click on "Disk Cleanup",

Third, Next click "Clean System Files",

Finally, check the "Previous Windows installation" file and click OK at the bottom, then wait for the cleanup to complete, and then delete the Windows (old) folder in C disk.