if you are slow moing the footage then you could expect the resultant files to be bigger, since the clips are longer. However if they don't play well in iMovie, it could be that you have rendered them out with a codec that your computer cannot handle. Motion gives you a whole heap of codecs to render out to, and its even easier if you have Compressor. Have a look at one of the clips that you know does play nicely in iMovie. Open it in Quicktime player and then get the info window open about it. There it will tell you what kind of compression it has been rendered in, what the pixel dimensions are etc etc. You will want to try and copy those settings with your new clips out of Motion.