Poor picture quality in finished file

I have edited some video I shot on my MiniDV camera. While editing in iMovie the picture quality is great. However, when I export it to a file format (I've tried .mov mpeg4 and full quality DV) there are artifacts everywhere. However, when I use iDVD to make a DVD of the same video, it looks superb, even when full-screen.

Given that the filesize on DVD (375Mb) is far, far smaller than the Full Quality DV (1.3Gb), why is the picture so much better on the DVD? The only difference I can see between all the file formats is that the screen size gets bigger in Quicktime as the filesize increases. It makes no sense to me - the DVD picture quality is superb but every format that plays on a computer is terrible.

Thanks for your help

Jack.

I've tried all of the available formats in iMovie and they all look naff. The biggest thing I notice is that the text titles look very strange due to the compression artifacts, even when I save as full quality DV.

Any suggestions?

Thanks