When I was a full-time IT industry analyst, one of the things that regularly annoyed me was overly large tables at dinner. A lot of factors were in play including the fact that many analysts felt very strongly about being at a table with the "important people" (tm) in the room. But if you were at even a round table seating 8+ people, it meant in practice you only talked with a few people who were reasonably adjacent.
I would say a 4-6 person table is about the largest where everyone can be talking with all the other people.
I suspect that it's more about the geometry of best fitting people into the space. It allows the right amount of room for centerpieces and bread bowls and such. At least as far as the caterer and wedding planner are concerned, conversation is the last thing on their minds.
Those tables are intended for eight comfortably. They stretch to ten. You can do twelve, but everyone is gonna be cramped.
It's a size and shape that's so omnipresent one expects that it's some combination of geometrical efficiency, ability to pass/reach things, some flexibility in self-organizing conversational groups, and probably some other things such as not being too large to setup, teardown, and store.
Could be - making sure that everyone at the table always has a choice of conversation to listen to, instead of being trapped in one they have no interest in.
I doubt it's that explicit, though. Maybe it's just worked out better that way over the years, without anyone fully realizing the why?
Six can be a bit of magic number for a table in a larger crowd, because people tend to be in groups of 2 or 4, so a table for six (apparently) means you’re inviting/ causing groups to mix.
I would say a 4-6 person table is about the largest where everyone can be talking with all the other people.