Can you try both side by side without the cap to see how much difference there is in drag? That way you can isolate whether it is indeed the diameter of the opening or it's the fitment of the cap and tip. I suspect that a lack of ports at the lip that the Ti has is the main cause of the restriction. The lip of the M tip is flat as well as the cap so there's not much room for air to get by.Following up on this, I tried leaving a very small bit of room between the M's cap and tip (i.e., not fully inserting the tip into the cap) to see if it was the lack of notches, but that didn't make a noticeable difference.
Leads me to believe it's that smaller inner diameter of the M tip, which leaves about half as much space around the condenser as the Ti tip.
I'm betting that the reduced diameter mostly for strength since the O-ring grooves and cooling fins create stress points in the steel. Ti doesn't have issues with stress risers at corners and notches so it can be made thinner. Also consider that the strength of the Ti is greater in general compared to 316L. We might not notice a difference in the final product but the extra strength would be important during machining.