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Input File:

  • Kind: MPEG-4 File
  • Size: 305 MB
  • Dimensions: 640 x 360
  • Codecs: H.264, AAC
  • Duration: 52:07
  • Audio Channels: 2

After using iMovie to trim a bit from the beginning and end of the file, I tried to export the file via File → Share → File… with the following settings:

  • Format: Video and Audio
  • Resolution: 720p (iMovie only offers me 540p as alternative)
  • Quality: High (according to an answer to a similar question this means H.264)
  • Compress: Better Quality

The estimation on file size shown by iMovie is 4.35 GB. My questions:

  1. How come the file size increases after I cut something from it, encoded both with H.264?
  2. Why does iMovie not offer me the input resolution as an output resolution?

I'd like to be able to trim a video and save it with roughly the same size/time ratio.

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4 Answers 4

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720p resolution is 1280×720, which equates to quadruple the number of pixels vs. your original 640×360 (360p) movie. This alone would account for a huge jump in storage size.

You'll want to output in 540p instead (960×540). However this will still produce a video with 2.25× the number of pixels, so the resulting video may or may not still be larger.

As to why you can’t output in 360p, this appears to be a design decision by Apple (possibly to steer us towards Final Cut Pro), because earlier versions of iMovie allowed more granularity. In fact, in iMovie 10.0, 480p was still available, but as of iMovie 10.1, it’s gone. I suggest you voice your concerns to Apple.

Lastly, I should point out that Low, Medium and High Quality settings all encode in H.264. They just use different encoding profiles/levels, with varying bitrates. From the figures you provided, I’d say your original video wasn’t produced using a profile equivalent to iMovie’s High setting. So you’ll probably want to try Medium or even Low.

If you still can’t keep your file size down encoding in 540p and using a lower profile, you can still down-sample it through Handbrake. However the upscaling-downscaling is sure to lower your video quality by some token amount. So it's a tradeoff.

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  • Are you saying Apple just won't allow me to preserve the dimensions of the original video? If yes, do you know how the extra pixels are generated? I can imagine an original pixel represented by 4 but not by 2.25, wouldnt that distort the image?
    – Human
    Commented Aug 5, 2017 at 1:08
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    It has been my experience that you have to either upscale or downscale if the source video does not match 720p, 1080p, or 2160p. I seem to recall getting more resolution choices by holding ALT in earlier versions, but this stopped working in a recent update and I don't know of any workarounds.
    – user11633
    Commented Aug 5, 2017 at 1:11
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    As far as upscaling, no, it won't distort the image the way you're thinking. It's just a digital zoom. Modern scalers are pretty good these days, and unless you're blowing stuff up by a huge factor you don't usually notice a drop in quality. Imagine that you're playing your 360p video in full-screen. It's the same kind of upscaling in action.
    – user11633
    Commented Aug 5, 2017 at 1:12
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    This only partially answers the question. The question asks why the file size gets bigger after shortening the movie. As you say, the change in resolution will quadruple the number of pixels, so with no compression you would expect a file size four times larger. But the OP is experiencing a file size over ten times larger. And given that compression is being used, there's no real excuse for any increase in the file size, since whatever is being done in the first place to create the extra pixels should be getting done by the decoder. There are more pixels but not more information.
    – Matt
    Commented Apr 25, 2020 at 14:06
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    the issue is you make a video that is 1080 in original source all looks and sound great..then you import to imovie and the size is bloated to like 10x the original size...so not sure your answer is addressing this...we know what we are talking about...it is not bout video quality..quality is great already question is what the hell is imovie doing that it bloats up size of video???
    – uberrebu
    Commented Oct 5, 2021 at 18:45
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I had the same problem as you. I searched all over and gave up on improving iMovie which is using x264 anyway. Instead, I used ffmpeg (https://www.ffmpeg.org/). It worked like a charm. My following command reduced the iMovie video size in average 1/5th.

ffmpeg -i input.mp4 -vcodec libx265 -crf 20 output.mp4

Note how it's using the better codec of x265. If you are considering EC2 Cloud9 like me, consider installing ffmpeg with this instruction (https://trac.ffmpeg.org/wiki/CompilationGuide/Centos).

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One of the possible solutions I came up with is you upload your video to YouTube and YouTube would help you reduce the size with good quality at the same time. Finally, you just download back the file from YouTube.

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  • Welcome to Ask Different. This does not provide an answer to the original questions which deal with iMovie file sizes. Please see How to Answer for info on providing a good, quality answer.
    – fsb
    Commented Feb 2, 2019 at 18:20
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I had a similar issue input file from my iPhone was 140MB at 1080p the edited version with 30secs removed exported at 720p with iMovie was 94MB.

I always use AVC (Any Video Converter) the free version and it exported at 720p using default settings this reduced it to 9MB with no loss of quality 1/10th the size!

Not sure why iMovie is so bloaty. I have to do this on a lot of videos at work as we stream them over the network.

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    Commented Oct 7, 2021 at 12:18

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