As a tall person I can confirm that using a short girlfriend's head as an elbow rest is a gesture of affection. I also do this with platonic friends, to mixed results. My favorite recollection is walking up to a college friend on campus who was talking to someone else, she introduced me and I did the armrest thing while joining the conversation. After a minute the other person said "Um, are you gonna..." And my friend said "Nah, he'll get bored with it eventually and I'm used to it. I have a lot of tall friends."