Recently I wanted to reuse an SD card that I had uploaded OSMC on for a Raspberry Pi. The card initially only showed 600mb of space. I couldn’t figure out where the rest of the storage went. I tried reformatting and didn’t get any more space. Here’s a decent guide on reformatting: https://gopro.com/help/articles/Solutions_Troubleshooting/SD-Card-Reformat-on-a-Windows-Computer
I kept digging and found an article directly related to what I was looking for, reformatting a Raspberry Pi SD Card. Here’s a good article on using the DISKPART utility on Windows 10: https://www.howtogeek.com/170794/ask-htg-how-can-i-reclaim-the-full-capacity-of-an-sd-card/
Also, if you happen to run the “Clean All” command expect for it to take a little while to finish: https://www.sevenforums.com/tutorials/52129-disk-clean-clean-all-diskpart-command.html