Mapped drives are not actual drives. they are folder/directories in a machine in your network.
you are saving their path as drive, so the free space or used space you will see is the space of that folder's drive.
Required c# code will be
using System.IO;
foreach (var item in drives)
{
Console.WriteLine("Drive: {0} TotalSpace:{1} FreeSpace:{2}",item.Name,item.TotalSize ,item.AvailableFreeSpace);
}