system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some countries have extreme population concentration in a single city?

To give examples of what I mean Greece has about eleven million people, three million of whom are in Athens. The next largest city is Thessaloniki, with about 800,00, then Patras with 200,000. Hungary has about ten million people, 1.5 million of whom live in Budapest, and 200,000 live in the next largest city, Debrecen. The Scandinavian countries have similar patterns, to use a different region.

I thought this might be a small country thing, but the UK's population rank goes London (10 million) Manchester (2.5 million) Birmingham (2.5) in terms of built up areas while Italy goes 2.6/1.2/1 by urban area or 4.3/3.2/3.1 by metropolitan for Rome, Milan and Naples, respectively. And Australia and Canada have very smooth population slopes.

I know that Athens exploded in size due to refugees pouring in from the Aegean region, but the patterns seems to hold even without these specific historical events. I'm assuming this has to do with respective histories of political centralization, but I'm curious what the precise mechanism is.
gpt
It's actually a really fun topic in urbanism! There's something called Zipf's law or rank power law that accurately models it. In most countries the biggest city is bigger than the second biggest by double and so on and so forth.

Essentially bigger cities are preferred because they are more productive, and since they are more productive they can offer larger salaries so they grow. However either due to bad supply of housing and other goods, or too much demand, or both,  living standards become more expensive as well. Thus therest reaches a point where this largest city is no longer worth living there, so people allocate into the second largest city and so on. Obviously this is a very simplistic explanation but it gives you the general gist.