I think size and complexity happen after the shift.

I think hierarchy arises once you shift from nomadic lives to settled lives. People begin to accumulate more personal possessions and competition for resources and choicest portions of resources grow.

But hierarchy doesn't have to be based on dominance, it can center on mutualism or altruism or even kinsghip. Look at birds in breeding colonies, for example. Or bats.

If it is dominance based, it need not be so rigid or based on maleness (patriarchy). Look at bees, for example.

We can see in all those examples, that it's not numbers that affect the hierarchal structure.

It seems pretty clear that it was staying still to cultivate rather than hunt and gather, abstracted ideas about how to divide resources such as money and property ownership, and then some sort of environmental upheaval that promoted invasion of territory that led to dominance hierarchy.