Firstly, are you sure your video is pixelating, and that what you are seeing isn't just interlacing?
Many people confuse interlacing, which is a totally normal part of how video has worked for 50 years, with a loss in quality, when in fact it isn't a quality issue at all. the article I linked illustrates interlacing, and explains how it works.
If in fact, your video is pixelating, and interlacing isn't the issue, let us know. But really, if you are using .mov footage, I'd recommend using the Animation codec as a standard, as it is lossless, so you retain the maximum possible quality, but with significantly smaller file sizes than uncompressed. Plus, it supports alpha channels, if you ever need to work with them.
Use the "Export using Quicktime" option in iMovie, and you will be able to use the Animation codec there as well, to ensure no quality is lost.