Its something I've notice my friends adult border collie do when my puppy was small. It seemed to be a positive thing, during playing etc. I think it seems like trust building, like the bigger dog is saying I am big, I can put your whole head in my mouth but I'm not going to hurt you. My boyfriend thinks it's a dominance thing. The border collie was very good at jumping and prancing over my pup without ever standing on him.