Windows 10 can at the bare minimum, take 20GB of disk space, and commonly takes up over 30GB when it's in daily use.
In my tests I managed to get Windows 10 to take up 8GB of disk space instead of the 22GB it was taking up previously.
This method doesn't involve installing any extra programs - the functions are all built into Windows 10.
1. Go to the start menu and click it so it's open
2. Type 'Command Prompt' - as you begin typing it should show up.
3. Right-Click the program 'Command Prompt' and choose 'Run as Administrator'
4. In the new window that appears, type "Compact.exe /CompactOS:query" (without the quotes) and press Enter. This will tell you if the system is already in the mode where it's trying to compress the files to save disk space.
5. If it's not compressed already, you can type "Compact.exe /CompactOS:always" and press Enter. This may take a while. In future, the disk space used by Windows 10 will be kept small because the system will be able to use the original files in their compressed state. You won't notice any changes to the way your documents look or open.
6. To reduce the amount of disk space Windows 10 uses even more, you can reduce the size of the hibernation file (the file Windows 10 saves everything to if you put the computer to sleep). With Command Prompt still open, type "powercfg /h /type reduced"
7. If you want to remove the hibernation file completely, you can use "powercfg /h /off" instead.
8. If you want to restore the default hibernation settings use this command "powercfg /h /size 100"
9. You may be prompted to restart.
The disk space used should be drastically less, leaving you more space for your own files.