The "number of people you've had under you" does provides insight into what a role entails, but there's still a lot of variance. What does seem to be uniform is that the "number of people you've had under you" determines whether a hiring company will think you’re qualified for a job. Companies simply aren't likely to take a "risk" hiring an outsider that hasn't been in a role with a similar perceived scope of responsibility. And since actual here's-what-you're-doing-day-to-day responsibility can vary so much, the "number of people you've had under you" becomes a proxy for that.

I understand the reasoning for why they put so much weight on the number of reports you’ve been previously responsible for, but it also explains why big companies stagnate. The people change but the thinking and strategy doesn’t.