salexander:Why not take your 20 minute sequence, double it to 40 minutes and (using the same QuickTime export settings) export it as before. If the result is still 1.5 GB then I'd say there is an issue. Otherwise, you may be running into essentially the maximum bitrate for QT MPEG-4 and not a filesize limit. Does this make sense?
Here's why I think it's a file size limit: I exported one minute of the same sequence with the identical settings and the image was perfectly sharp. When I export the entire sequence with the same settings, it looks terribly blocky and compressed. I increased the bitrate limit and quality settings and ended up with an identical file size (1.5 GB) and appearance.