There are many smaller democracies that don’t score well, such as most of South America, the Caribbean, much of Africa, etc.

Sure, I acknowledge that. I’m not saying that a smaller population guarantees a successful democracy, nor a social democracy, but I think it is one of the requisites. Those other things you mentioned are probably requisites as well.

Again, I think it comes down to simple math. A single representative can’t represent 600,000 people as effectively as 30,000. More people means greater diversity of thoughts and ideas, beliefs, ideologies, interests, etc. And that’s especially true if the people hold mutually exclusive ideas. For instance, a representative can’t represent both a white supremacist and black civil rights leader simultaneously. Their goals and world view are diametrically opposed. A representative can’t represent both at the same time, at least not on the matter of civil rights. Similarly, a representative can’t represent both a social democrat and a neoliberal capitalist simultaneously. Their goals are in direct opposition to one another. The social democrat wants higher taxes and a stronger social safety net, the neoliberal wants lower taxes and a smaller safety net.