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.


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.

  • 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 Aug 5 '17 at 1:08
  • 1
    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 Aug 5 '17 at 1:11
  • 1
    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 Aug 5 '17 at 1:12

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.

  • 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 Feb 2 '19 at 18:20

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