In my experience, I have only accumulated evidence for this notion and very little (none) against it.
In my days as a teaching assistant, I struggled through the times where I tried to initiate a class (of 25 to 30) discussion on whatever topic was in vogue that week. I knew that the same louder and more extroverted voices would be heard, and I would have to inevitably pick on those shier to contribute. I don’t even know if calling it a conversation would be appropriate since it is rather inorganic phenomenon in the class setting. This is the primary reason why my favourite teaching technique would just be to split the room into groups of three. The quality of the conversation increases significantly and I am able to go around checking-in to make everyone is on the same page.
Large numbers are probably the reason why “conversations” on social media are so poor. From twitter threads, facebook or instagram comments, it is a all a jumbled mess. Even if one person contributes to the conversation, it is unlikely they will respond to all, it not any of the replies to their message. I think that is the central problem, with a larger group, you invariably lose individual accountability, and therefore, can slip and and out of the conversation at will without disrupting the status quo. In a group of 2, 3, or 4, exiting a conversation will change the tone of it. But in groups of 10, 20, or 30, one person exiting is almost inconsequential.
If you are in conversation with a group of 8 or more, you invariably splinter off into smaller groups. It is just not sustainable to carry a balanced conversation with that number of people. It is possible, but inevitably, there would be people that feel unheard. Dialogue is a rather inefficient form of communication in that respect, it just does not scale well.