Like Crissy has mentioned about Adopus, the Macropus complex has a wide variety of species (look through my favorite book - if you have not already - and you will see a few really odd ones) attached to the group. Have you seen the green fluorescent dots like on O.briareus yet? Several of my pictures of Puddles would make you think O.briareus but he definitely was not.
There is the possibility that this one is so young that the arm ratio has not yet reached adult proportions but I kind of doubt it since my O.briareus and O.mercatoris hatchlings both were proportionate when they were very small. However, this is a pelagic born species and they start out not looking much like the adults (unlike the two large egged ones I have raised. I have not had the opportunity to see a small egg hatchling yet so I am going by photos but Monty is likely a small egg species and the proportions on his arrival have not changed much).
There is the possibility that this one is so young that the arm ratio has not yet reached adult proportions but I kind of doubt it since my O.briareus and O.mercatoris hatchlings both were proportionate when they were very small. However, this is a pelagic born species and they start out not looking much like the adults (unlike the two large egged ones I have raised. I have not had the opportunity to see a small egg hatchling yet so I am going by photos but Monty is likely a small egg species and the proportions on his arrival have not changed much).