How large/long is each movie?
What method did you try to use to export? Which one of these did you try:
I get the best results using export using QuickTime and then use Expert Settings under Options (and the 1280x720 is 720p HD or as close as iMovie will come to doing HD):
You can adjust everything separately; I've also found that H.264 compression is the least "damaging" to HD video.