I did some shooting in HD and I edited it with ease with iMovie '11.

Once I edited my video, I exported it and I was shocked: lots of compression artifacts, terrible to watch in HD! In the shadows, the faces of people, there were those huge ugly pixels.

How the hell is this possible??

I went immediately to see the original recordings, and those artifacts were not present. So I went to see the files that iMovie creates as intermediaries for faster processing, and those were perfect.

So, I think, I've chosen a low compression bitrate. I choose an absurdly high bitrate and I got with a 1 gigabyte-per-minute video. With the usual compression artifacts.

Out of curiosity I import the project in Final Cut Pro X, and export it with the same settings

I get a small file, without those ugly compression artifacts.

Here is a comparison of the same frame with the same export settings from iMovie and Final Cut Pro X:

Here is the original video

My conclusion is that iMovie intentionally reduces the quality of exported video to avoid a professional use...