I'm not sure "economy of scale" is the appropriate expression but I understand what you are trying to say. But if you give the statistic differently, it makes a little better sense. Take for example, my hometown that I mentioned earlier.
It has a population of about 8,500. There was one murder in 2010. That means, in a way, that you have one chance in 8,500 of being murdered. The county where I live now has a population of about 1,100,000. There were 22 murders in 2008, the latest year available, apparently. That gives you a one in 50,000 chance of being a murder victim. So, although there are 22 times as many murders here, you have a better chance of being murdered in my hometown, where I would move at the drop of a hat. So in other words, the crime rate per 100,000 makes no sense if there aren't that many people there to begin with.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.