Well I am going to disagree. I have 2 large, 1 XLarge, 1 small, 1 tiny. All dogs know that there is a difference based on their sizes. The small ones are allowed on the couch and the beds, and the big ones are not. There is an understanding and there is never any debates nor jealousy.
We treat all our dogs relatively the same, but this is one exception. Is it right? It's the way of this household and we have no issues with the dogs argueing over who gets to sit or sleep where. It's accepted and they find their relevant spots to lay.