> If you look at the geography, if you look at the political climate, just at the fundamentals it's not clear why Vienna is a large city

Sits on the Danube and was the capital of the Austrian Empire, so very important all throughout the 1800s. I'd say strategically located in Central Europe.

Then in the second half of the 20th C it was the gateway to the "East Bloc", the place where east meets west in terms of diplomacy, economic and cultural exchange, and of course espionage.

It has played a neutral role similar to Switzerland, which is why OPEC is headquartered there. It's also one of the UN's four major sites worldwide (along with NY, Geneva, Nairobi).

I spent a couple of years there; nice place to live; would happily return.