quick time in maverick,

After installing Maverick 10.9 when I open a .mov file automatically converts it (don't know why) into anothe .mov file much smaller then the original.

MOV is a generic file container which may contain any audio and video compression formats compatible with the codec configuration of the system on which the file was created. QT X v10.3 now looks at these compression formats and decides (rightly or wrongly) whether or not the file should be re-encoded for better QT compatibility. Since you have not indicated what compression formats are used in your MOV files, it is difficult to say exactly what is actually triggering your specific conversions. As to the size of the final file, the QT H.264 preselt employs context adaptive settings designed to produce a targeted level of quality based on a theoretical "average" level of quality for the sourced video file. Depending on the relative efficiency of the source video codec in comparison with H.264, the final file could be smaller, larger, or equal to the source file.

Also iMovie hangs and crashes.

Can't help you here. May be due to the same sourced compression formats or something entirely different.