Trading seems a good explanation, yes. A trading expedition is a big deal, you probably want cram a ship-ton of resources together and exchange all your obsidian for salt, or grain for copper, or whatever, in one go. You don't want to mess around making massive journeys between hundreds of little villages. You want a large, famous, easily-spotted trading destination that has all the goods ready in a big convenient pile.

Most of these sites have astronomical/calendrical orientation. I would say the definition of trade has to include knowledge and techniques. I can imagine a culture developing a steady state around the seemingly consistent movement of celestial objects and slowly losing it’s cultural center has things shifted.

I find interesting that astrology is based on the rising of the sun in position of the planets which is distinct from a lunar calendar.

What sort of cultural shift would that change create?

Given how basic their lifestyle was, I'd say you hit marginal returns at tens of individuals. Like I said in another comment, a stronger reason would be required for people to want this scale, and I can only see warfare/peacekeeping and matchmaking as reasons.

> Given how basic their lifestyle was, I'd say you hit marginal returns at tens of individuals.

You might have a circular argument there.