No, the rio is not too large. For the berlin, the more flow through it, the better it works. The problem with the Berlin is salt creep in the venturi leads to incosistent skimming.
A lot of people add the Kent venturi and say this makes the skimmer.
In my opinion most of the top-end skimmers simply aren't designed with consideration of the theoretical design requirements for good skimming with the exceptions of air injection (lots of small bubbles). It appears the main design criteria for beckett skimmers, for instance, is that they be short (to fit in under tank sumps), inject a lot of small bubbles, and that they look pretty.
We don't know what skimmers remove from the water. We don't even specifically know what we want to remove from the water other than saying "waste material". We don't even have a clue on determining relative skimmer efficiency.
I remember CWA asking a simple question in another discussion forum. To paraphrase CWA's question: "aren't short body skimmers with high water turnover rates contrary to the basic principles of good skimming?"
-Sci
(CWA, I hope you don't mind me loosely quoting you. You asked an obvious and good question and I've never seen a good answer.)