Is it safe to assume the syntax you are using is more like:

_movie1.wmv (640x360)
_movie1.wmv
_movie1

I assume so, just wanted to be clear for other users. Not sure why leaving the extension off would make a difference. The movies are all fed to the Windows Media Player's video engine, so it may be that the WMP is confused by the not knowing exactly what format of video it's being asked to play. As for jumps from normal to larger sizes, I can't say. You might try experimenting with different video playback codecs.

See also:

http://www.empirisoft.com/support/showthread.php?p=3997