I suggest 2 actions to get a clear picture of what is on your disk:
A) Run WinDirStat which will give you a complete picture regarding all system and user files on the disk.If you have big colored chunks on the picture, you can click on them and it will tell you above what it is.
B) Run the command below in an elevated command prompt. That will tell you the allocated space for shadowstorage (that's where system restore points are being kept). This space is not shown in the WinDirStat list.
vssadmin list shadowstorage