There are a huge number of reasons why someone might have a preference like this, and almost none are going to be the result of careful, logical reasoning, unless someone has really sat down and analyzed it.

Exactly my point. Preferences may exist for different people for different reasons and as long as those reasons and preferences don't harm anyone it should be accepted.

Anyway, I don't really know what you're asking for, with studies.

You said thinness is valued in women since it's seen as indicator for being unimposing. I'm asking where you got that from. In my interpretation more than being petite, a shy nature or introverted behaviour is seen as unimposing. So behaviour over body type. I would argue that most people have the "healthy and fit" reasons more than the sexist one.

But like you said, no one can pin point a single origin for liking a certain feature and hence a preference's origin should not be used as an argument against having a personal preference since no one actually knows which origin is the real one.

The point is whether a particular preference is unnecessarily limiting.

How is it unnecessarily limiting? If people like thin women because they think they are unimposing then that is sexism and thus harms others. My post specifies that having a personal preference is fine as long as it doesn't harm anyone else. So even if the origin you gave was right it wouldn't make a difference.

Also, you didn't answer how a bad origin would make having that preference morally bad as well since we can't change what we're attracted to.