I think its also important to note the human aspect of it all. The saying: "You can only keep a secret between two people, and even the one is safer."

Is true even here. So the larger the group the easier it is to sneak in as a third party eavesdropper.

Even with best security encryption the screenshot leakage will increase per person in the group as well. 

 Right. Encryption protocols can only ensure security from a technical perspective; they fundamentally cannot address the issue of malicious actors within the group. 

 After a critical size the chance of a malicious actor in the group becomes the weakpoint rather than the cryptography.

This MLS is very cool however and will be very nice to see implemented. It will be the strong point.