iMovie Export Quality Suddenly Goes Bad

Aside from the difference between the estimated file size and the exported file size, did you visually note any difference in quality? If so, instead of the Better Quality custom setting, export at Best Quality (pro res).

iMovie unpacks for editing videos that are imported into it. That makes the file size bigger in most cases. On export it is compressed to Mp4 or .mov, depending on your export settings. When you export at Best Quality (pro res) you will get a .mov file that is the most uncompressed and 4x or more the file size of the Mp4 format that you get with all export settings other than Best Quality (pro res) that is the best quality. Pro res has a much higher bitrate and thus larger file size and better quality.

If you still feel that iMovie is randomly changing your export settings, try opening iMovie while holding down the Option and Command keys and selecting 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. Now see if your exports are being exported at the proper settings.

It is the compression, frame rate, resolution, and bitrate that determines file size. The user makes those choices by choosing the export settings.

-- Rich

There might be a way to back up or download the iMovie DMG, but I have never had occasion to try that, so can't be of any guidance there. However, the way to reinstall is to first put your present iMovie app in the trash, but don't empty the trash. Then go to the app store and redownload. After the redownload successfully completes, you empty the trash of you old iMovie app. So, you would not be using up any more space.

You need to do some trouble shooting to find out why iMovie keeps corrupting. Try booting up in Safe Mode and, after everything loads, immediately reboot in normal mode. See if you can now export properly.

You already tried deleting preferences as I described earlier in this thread, but try doing that again as sometimes the second time works.

Also, if you are exporting directly to You Tube, there have been some issues doing that with Catalina. If that's your situation, try sharing your movie to your desktop and from there dragging it into You Tube's upload window.

Another thing is that iMovie support export of 4k 30fps, and 1080 60fps, but not 4k 60fps. Possibly that's the problem.

-- Rich

A reinstall was the next thing I was going to suggest. Often that cures problems.

If you have time, let us know if it worked.

-- Rich

The app estimates of file size aren't necessarily accurate.

It's puzzling that your first 4k export turned out fine, but later ones not. Is there anything different about your first clip versus the ones that were unsuccessful? What format are these video clips? What frame rate? Were they taken by an iPhone or by a camera? Were they recorded by the same device or different devices?

Sometimes if you upscale a project's resolution by more than one level (say, 720 to 4k) that can result in decreased quality. That's probably not your situation, but just thought I'd mention it. The resolution and frame rate of a project are set by the resolution and frame rate of the very first clip that is added to the project.

-- Rich

At this point you might benefit from contacting Apple Support and relating your problem to them. They are pretty savvy. To contact Apple Support click on the "Contact Support" item in the upper right hand corner of this forum's screen. Navigate your way to a telephone number and call.

-- Rich

Glad that it is now working.

-- Rich