You have threshold effects.

Assuming equivalent distribution, the higher the amount of people, the higher the amount of extremely difficult cases.

Sure, the US is going to have more very difficult cases because it has more people.

Let's skim those very difficult cases off for the moment: why can't the US make better use of concepts like restorative justice and rehabilitation for the rest of the criminal justice population?

It's cheaper; it's more effective to reduce crime; it's better for the children involved; etc etc etc.

> Assuming equivalent distribution, the higher the amount of people, the higher the amount of extremely difficult cases.

So divide the jurisdictions to where they're manageable. We already do this - each state runs their system independently, just as each EU member state does instead of having a single European prison system.

People can travel.

The larger the population, the more inmates(in total number) there will be. Therefore, its organizational structure becomes harder to manage and prone to bureaucracy.

The more inmates in total numbers yes, but the US have almost 10 times the number of inmates per capita Norway does. The state with the lowest incarceration rate still has double Norway's incarceration rate (Maine, 148; 25% the population of Norway and a similar density).

So subdivide it in nice manageable chunks and manage them separately.

This argument of scale comes up time and time again in these type of discussions despite being totally invalid in most cases: In most instances that are up for discussion it is not necessary to build one single large system.