iMovie generates big unknown files

iMovie generates big unknown files that won't away, taking up all my storage spaces

I don't use the forum here a lot but this time I really need help. I cannot solve the situation by myself and I want to see if anyone here can give me an answer.

It begins this morning. I was using iMovie to make some videos (30~mb of original medias and 13~ minutes of them loaded in the timeline). I did something usual with it. I changed the clips' speed, reduced noise, went through all the clips in iMovie to familiarise myself with the contents (I didn't shoot the videos), etc.

However, soon the "disk storage full" notification interrupted me. It often happens to my MacBook as some apps are generating too much additional files. I cleared my storage earlier today and there should be 4gb left in my Mac, but after using iMovie, literally none is left. After restarting the computer, there are now 800~900mb left. I was desperate. I have JUST cleared the space this morning by throwing everything on dropbox. 

I checked everywhere but nothing was found. I first checked the iMovie library (the one in the Movie folder) but it only took up around 30mb. Next, I checked all main folders under my own account, but again, nothing suspicious. It was usually the library's fault, but it only took up around 2gb and other folders, such as Google drive and desktop, took up even less. 

Assuming the unknown files are generated in one specific place, my own user folder should be off the table, same to the one of another user since her account isn't opened during the incident. After further investigations, the following are NOT the possible causes of the sudden file growth:

• iMovie related apps (the app itself, the library app and the project app)
• library folder A (storage/users/*my user name*/library)
• library folder B (storage/library) (takes up only 3gb and definitely not all suddenly generated)
• main folders under my account (storage/users/*my user name*/*everything here*)
• main folders under other account (my sister's and the guest one)
• the shared folder between users
• application folder (storage/application)

Possible causes includes:

• the system folder (storage/system) (personally thinks it seems important, so I didn't do anything with it)
• the hidden folders in storage (storage/*hidden folders*)

Extra info:

• this issue has happened before in iMovie app, but less terrible. It did clog up my storage and I don't remember seeing the files go away.
• it also happens when I played certain games like Cities Skylines and Minecraft, but the files disappeared and freed my storage space soon after I turned the app off.
• It can possibly be downloaded files instead of generated files that caused the issue since I edited the video with Wi-Fi on. Sadly, it also means that it could be virus, but i don't think there'd be virus that are activated by iMovie, as a reputable app under Apple.
• I think that the issue could be caused by iMovie or some hidden applications activated by the iMovie app.
• I've looked up the internet but not a single source of information can answer me how the files could be generated when using apps, which is strange because it happens to me a LOT.

I understand that it is quite a long post and thank you for going through all the words to help me. I wish someone could give me an answer as soon as possible so that I can go the folder and delete the useless additional files. It'd be very useful since I'm sure that those hidden file is definitely bigger than 4gb. If you encounters the same issue but don't have a solution for this, you can also leave a comment about your experience here and see if we can solve it with the extra info.