Why is iMovie Quality so Low?

First, you need to go log into your youtube account and set these videos to 'public', otherwise they can't be viewed by others than you.

––––

OK, it's possible that it took so long for Youtube to get their whole copyright-infringement scanning and alternative formats conversion done.

The MP4/H.264 versions of both videos have different specs:

#1: Frame size: 640x480, frame rate: 29.92 (~30), bit rate: 1,138 kbit/s

#2: Frame size: 640x360, frame rate: 25, bit rate: 593 kbit/s

I'm assuming that the original footage was shot in 640x480 (4:3 aspect ratio). Video #2 is in 640x360, which is 16:9, so the 4:3 video has been placed within a 16:9 project, which makes it smaller and gives the movie black bars left and right.

Try changing the aspect ratio of the project in iMovie to 'Standard 4:3' (press Cmd-J for the project properties.) This will also allow you to choose the 640x480 size when you upload via Share>Youtube.

When you create new projects, make sure you set the properties accordingly to the recording specs of the video files. If you don't know them, open the files in QuickTime Player and press Cmd-i to view their specs. Look for 'Format' (to determine the aspect ratio) and 'Frame Rate'...

The quality looks very similar. When you export using iMovie you have the option of exporting it with different quality ratios. You can only export as high as your camera shot it. So if you shot the video with a low quality camera, then it will look low quality and vice versa. You might want to double check what size you export with, and if you have the option on exporting in HD, do that. The larger size you export with, the better it will look. You can also try editing the contrast of the video to make it less grainy. Just double click on the clip when it is in a project and it will give you options for editing the video picture. I hope this helped!