What does "bedroom eyes" mean, from a physical standpoint, if anything?

Dilated pupils or heavy lids are not a part of the definition of “bedroom eyes”. More generally, there is no one specific physical aspect that is essential to he meaning of the expression “bedroom eyes”. Any “way of looking at someone that shows you are sexually attracted to them” (Macmillan Dictionary) can be called “bedroom eyes”.

Marilyn Monroe’s famous “bedroom eyes”

This is not to say that dilated pupils cannot be “bedroom eyes”. In fact they can, and for good reason: one cause of dilated pupils is sexual arousal. See Wikipedia’s article on the subject, “Mydriasis”, under the subsection titled “Effects”. In scientific studies, subjects rate images of people with dilated pupils as more attractive.

(Image from the article “Eye Signals”, published in Westside Toastmasters, For Public Speaking and Leadership Education.)

A tea made from the poisonous plant Atropa belladonna has been used by countless women to dilate their pupils or “darken the eyes”. Hence the name of the plant, bella donna, Italian for “beautiful woman”.