I can replicate this using the movie linked below on 10.4.3 and QuickTime 7.0.3.
What seems to be happening is that the movie refuses to scale horizontally and only scales vertically when adjusting dimensions numerically in the Movie Properties window.
Neither percentage nor pixel scaling works and the behavior is the same whether or not 'Preserve Aspect Ratio' is checked.
Workaround: Drag the lower corner of the movie window and you can resize the movie and it will retain aspect ratio. Option-Drag resizes proportionally to 1/4 size increments also retaining aspect ratio.
Same happening in 10.3.9 + QT 7.0.4 very annoying.
No problem using QT 6.5.2
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Version: 7.1.385 / Virus Database: 268.3.5/303 - Release Date: 4/6/2006