Spike up system data storage usage after failed exporting video file to MacBook from iMovie

I created a 1 hour video and wanted to save the video on MacBook.

It was failed while it was downloading, because of not enough storage.

Even though there was enough space when I checked it before start downloading.

So I wondered why there was not enough space and checked storage.

Well, my system data took a lot of space ridiculously.

Yeah System data usage has spiked up while my video was downloading.

It was my first time using iMovie and creating video, so I do not have much knowledge.

Even though I do not really how the process the video, however, I guess iMovie uses system data while it process creating video and it needs a space as much as video file size?

so eventually, device needs double amount of the space of video size?

I had barely enough space to save the video file when i tried to save it on device.

Like, my device would be full 98% if video saved on my mac.

Does anyone know the reason why it was failed and my system data storage takes so much space right now after failed to save video?

And, I would like to know clear the system data storage if it was because of the video.

Thanks