Comment by Olreich
Once organizations get to a sufficient size, increasing your "scope" is the only metric left to compare. You could compare revenue, but the easiest way to get more revenue is to increase the amount of work that falls under your purview. You could compare profitability, but then you encourage everyone to make the most expensive products they can get away with and your company fails. You could compare productivity, but there is no scientific way to do that, and funding the research required would bankrupt you and your company fails. You could do it by vibes, but the snake oil salesman will sell you garbage and your company fails. You could do it by seniority, but then you stagnate and your company fails. You could do it at random, but then none of your managers would bother trying and your company fails.
Do feel free to suggest a better way to compare two managers that doesn't fall into worse situations than "scope".
So, there is no objective way to measure performance of a manager ? It sounds similar to measuring the productivity of developer.
Measuring scope to evaluate performance is like saying developer who works on most number of tickets/bugs is the highest performer.
We know that's not true and we also know how it can and will be fudged(w.r.t managers its increasing un-necessary headcount).
If we are able to come up with a solution for developers it shouldn't be too difficult for managers too.