I have Windows 10 on a Dell* and I could not for the life of me figure out why my C: drive space was disappearing. I tried disk cleanup, deleting programs, all of that. Turns out in the documents folder WeChat was storing away 25GB (!) of files. You can delete all this, just go to:
C:\Users\[your user name]\Documents\WeChat Files
And delete everything. Next time you open WeChat you lose your chat history and images on your computer but it works fine. The graphic icons/avatars of people will be grey squares, but the images will reappear as soon as they talk to you. If there were documents you needed, just go on your phone, find the chat where they were sent to you, and resend them to yourself.
*Dell is the shittiest laptop I have owned in decades. It’s expensive, clunky, and ugly and has an irritating ‘Support Assist’ program that is a massive waste of time. Never again. Having said that, this particular problem was not their fault.