Homogeneity makes it easier to build a governing consensus; people can believe there's an optimal policy benefiting everyone, "because we're all so alike", rather than a zero-sum contest between disparate groups.

Norway has gotten quite a lot of immigrants the last couple of decades and homogeneous might not be a key word anymore.

A couple of friends of mine are Bangladeshi academics. They are the nicest, "squarest" and most hard working nerds I've ever met. I had lots of fun sending them a reference to the news last year -- about problems in Oslo with Bangladeshi street gangs... :-)

But sure, the problems with planning and controlling a society isn't proportional to the population, more like the population squared.

It should also be less problems with fewer variables.