Recently upgraded from OS X (10.5) to OS X (10.6). .MOV files played fine in Quicktime before the upgrade.
I recorded the movie using a camera, and the file plays on the camera as a movie... I'm stumped.
Then I suggest you compare the size of the playing file on your camera with the size of the non-playing file on your computer—they may not be the same. As QTKirk indicated, 40.37 KB seems unreasonable for any normally encoded non-anamorphic 1280x720 MPEG-4/AVC file more than a few frames long. For comparison purposes, a typical 1280x720 JPEG photo compressed to its smallest file size setting on a Mac would have a file size on the order of 100 KB. Other ways to look at your file would be...
a) if it is one second long, then its data rate is on the order of 322.96 Kbps
b) if it is two seconds long, then its data rate is on the order of 161.48 Kbps
c) if it is three seconds long, then its data rate is on the order of 107.65 Kbs
etc., etc., etc. which is unheard of for any reasonable amount of quality for 1280x720 HD dimensions in this compression format.