It's the natural outcome of stack-ranking systems. If managers know that the bottom 10% of their direct reports are going to be shown the door, protect the whole team by hiring people just to rank them in the bottom. If a manager oversees 200 people, is more likely ~20 of them will be bad hires and can be let go, but if a manager only has five direct reports, losing just one, even if that one isn't all the great, means losing 20% of the team.

On that note, I wonder how these forces might work to fix ideal team size to align with these flawed incentives.