I have fairly large ears too, and yet I am not getting much isolation from my Merlins. But I wonder if something as simple as the curvature or thickness of the canal portion makes a big difference here.
When I look through the vent hole, I can see the exposed armature drivers inside the shell. Those connect to a solid acrylic section that makes up the tip of the IEM. As far as I can tell, that little section is what does most of the work as far as sound blocking. Maybe yours is more substantial than mine?
Another idea: maybe your vent is partially blocked by your ears once you get the Merlins inserted. Mine also faces down but not so much that it is covered by the skin of my ear. Maybe that angle determines isolation to some degree?