I couldn't write Thessaloniki correctly.
Osaka didn't came into my mind, same as Alexandria.
"Brussels [...], officially the Brussels-Capital Region[...] is the capital and largest city of Belgium and the de facto capital of the European Union (EU)."
"The city has a population of 1.2 million and a metropolitan area with a population of over 1.8 million, both of them the largest in Belgium."
No, that is too cruel -- a hint: perhaps two extremes of Europe.
I suspect that both were early capital cities in their effective countries (economic regions).
Citywise I consider Birmingham second after London. Although there are two cities in London considering the British city definition (place with a catthedral) - City of London and City of Westminster. London divided by two is still bigger as Birmingham I guess. Very confusing this stuff. Furthermore, I think it depends on how you count if Greater Manchester or the West Midlands are the bigger urban area.