Cute picture:) I think it is probably dominance. Does she only do it when they are playing? It could just be like a "hahahahaha! I pinned you down!" thing.
This is definitely dominance. I discourage this with my guys....generally, though, it only happened when they were very young (under a year). They no longer do this. Guess they've figured it out amongst themselves, so they don't have to posture any more.
Our little one would do this to the bigger one but she cqn't instead she walks aliong side of her with her front paws on her bqck trying to hold her back. She is always the dominant one.
Our female is definately the dominent one, but she is also smaller. So instead of standing over our Labradoodle, she puts her paws on his back and "walks" beside him now and then. They have been together for 3 years and never a fight or obvious aggression, but I think that's because he knows she's the boss!!!