Why does iMovie keep consuming disk space when I am not editing ?

You describe rather an odd situation. I am not exactly sure what procedure you were doing. So, I can only speculate as to the cause.

iMovie will do background tasks, such as rendering your edits in real time. Your rendering may have aborted at 453MB because you were running out of disk space. You had 63MB of rending files, which is quite a lot. The "task" that the error message references probably is your edit rendering or exporting process (if you are trying to export). As Ian says, a 3 hour movie should be much larger than 253MB. When you cleared out some space, the rendering may have started up again unless you had done a Force Quit of iMovie. That might explain why disk space was decreasing, and may have been doing fine until you quit iMovie.

In any event, try returning to iMovie and commencing again whatever it was you were doing that flagged the insufficient space issue. Let the tasks finish out.

Also, try these procedures and see if they cure the issue:

Deleting Preferences

Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears.

iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Safe Mode

You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.

Resetting the System Management Controller

Try resetting the System Management Controller (but no longer available for the new M1 computers). It is a simple procedure that requires rebooting while holding down certain keys. The link below describes the procedure.

https://support.apple.com/en-us/HT201295

Resetting the NVRAM or PRAM

Try resetting the NVRAM or PRAM:

https://support.apple.com/en-us/HT204063

-- Rich

There are a number of discrepancies in your description.

463MB is very small and 3 hours just does not make sense. A 3 hour video will probably be around 10GB if not bigger.

It sounds as though iMovie may be converting the file to ProRes which would account for the massive use of disk space but I have no idea what you have done to make it start converting.