iMovie file exports are massive

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Level 1

8 points

iMovie file exports are massive

Yes, there are a lot of questions and answers that I found about this - both here and by googling. Generally, the answers are long lectures about how file size is determined by compression, video quality, length, etc... I understand all of that. But there must be some simple way of fixing this problem.

I have a file of a zoom. it's over an hour long, 1680x1050, H.264. It's 164MB.

I want to trim it down to make it shorter. I import it into iMovie (10.1.14, running on my MBP with OS 10.14.6). I cut out half of the video. I export it as 1080p, low resolution. The resulting file is over 860 MB.

So I would like to use iMovie because there are a few specific edits that I need, but mainly I'm throwing away half of the data. Why is there no way of saving it, with those edits, without massively increasing the file size?

MacBook Pro 15″, macOS 10.14

Posted on Dec 8, 2021 7:26 PM

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Answer 1

Best reply

Rich839

Level 8

47,316 points

Dec 9, 2021 11:29 AM in response to mar10029

I'll repeat what I said earlier. You likely are exporting a far less compressed file than you originally imported. That's just the way the software algorithm works. Zoom's 164MB file size for a one hour video is very highly compressed. Zoom may have a necessity for highly compressing its videos., or its video file size would have been way larger to start with. So it's really not an issue of "fixing this problem". Each software compresses to suit its purposes. Zoom's compression rate suits its purposes for displaying over the internet, but may not be suitable for your purposes when expanded out for video editing . iMovie unpacks video clips that are imported into it so that they can be edited. Highly compressed video is not optimum for editing. After unpacking , iMovie may export it out at a lower compression rate than the original clip. Hence, bigger file size and likely better quality. That's a good thing unless file size is critical. If the latter, then your recourse is to use the methods that I described in my earlier post, and adjust the export settings to more highly compress the file. Try 720 instead of 1080 resolution for example. .

-- Rich

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Answer 2

mar10029 Author

Level 1

8 points

Dec 9, 2021 6:45 AM in response to Rich839

Thanks! I tried doing what you suggested and set the custom quality to the very lowest setting and it actually produced an even bigger file...

The output was 1.06 GB for a video that's the same dimensions and half the length of the original mp4 video, which was a 164 MB file.

Of course, I know that I can make a file smaller by reducing the dimensions, but why does the simple act of essentially throwing away half of the data make the file almost 10 times as big?

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Answer 3

mar10029 Author

Level 1

8 points

Dec 9, 2021 12:14 PM in response to Rich839

Right, I understand that. I guess my question was if there was a way of ending up with an edited version of the original file that wasn't significantly larger.

In other words, if we are expanding a compressed file, we are adding no new data. So there is no reason why a file with half of the original data should necessarily be ten times the size. And since we are adding no new data, there is no advantage of less compression, right? It can't be better quality than the original, which was acceptable.

Maybe iMovie simply doesn't allow this, but I guess what I am looking for is a way of re-applying the original compression that zoom used to produce a one hour 1080p video that only took up 164 MB. Perhaps you know of a way to do that. I did try the method that you described in your earlier post, as I mentioned. I mean, eventually I could come up with a smaller file size if I reduced the dimensions severely. I could also just use QuickTime to trim the original, which isn't a great solution but maybe it's all I have...

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Answer 4

Rich839

Level 8

47,316 points

Dec 9, 2021 1:53 PM in response to mar10029

When you decompress a highly compressed file you are not exactly adding new data but you are displaying more of the original data. The more independent frames that you have in a video, the better the quality. I'm not an IT expert, but in a simplified way, when you compress a video you might have one independent frame that contains all of the data, but the next 200 frames are not full data frames. Rather, the format's algorithm refers back to the original frame, or some interim frame, and calculates the change that will occur to display the following frame and substitutes a code that displays only the change and refers back to the original full frame for the rest. That reduces the size of the video because each frame does not display all of the data but only a code created version. If the video is highly compressed you have a great number of code created frames instead of full frames. Tiny errors can happen in the calculated changes that reduce the quality of the video. When you decompress it by using a less compressing codec, you release the compression and display more full frames. So you are not adding new data but are displaying more of the existing data.

If anyone on here has more expertise than I have in this area, feel free to chime in.

You might be able to identify the Zoom format and settings and then convert your video back to it with a video converter like the free download, Handbrake.

-- Rich

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