1440x1080 is HD (sort of) anamorphic. The proper display image size would be 1920x1080, which is 16:9 aspect. If you're using export with quicktime conversion it's because you want to change the codec and/or scale it. So, chose your codec and set the export size to a 16:9 aspect, commonly 1920x1080 or 1280x720 or 960x540 etc.
If you don't want to change the codec, just use export quicktime, self-contained (or not if you just want a proxy) and go.
The 850x478 thing is just quicktime trying to be smart and taking the current display size setting and assuming that is the size you want (you probably don't). In fact, don't use numbers that are not divisible by 4.