My personal theory is that it's straight up impossible to have a non-authoritarian government involving more than Dunbar's number of people (counting both the governors and the governed). For larger numbers, the best you can do is a multi-level federated approach where the governing assemblies on each level have no more than that number of delegates from the level below.
Then you're likely run into a problem with communication. You can see in basically any large company: communication gets lost, misinterpreted, you end up depending on many other entities to do anything etc.
