Frankfurt

Why is the main airport hub in Germany when it is not the biggest city?

Not sure, but I guess the same applies in the USA... Why are ORD and ATL the biggest hubs when they aren't the biggest cities either?

Probably due to the fact that's its Germany's financial hub, and some argue the EU's.

Germany is an unusual country in that sense, when West Germany existed, the capital was Bonn. The financial centre has always been Frankfurt, and I'm not sure what status Berlin had during that period.

Its much more distributed than the likes of the London centric UK, with the car Industry being based around Stuttgart, and a bunch of industries based around Munich and other's having their areas of expertise.

So its no real surprise that the main hub is not in its biggest city, or even the one containing the parliament (used to be different, Frankfurt am Main is 5th largest city).

I seem to remember reading somewhere that Berlin was closed to German airlines until, relatively, recently. Connections from Berlin are still, in my opinion, quite poor when flying international. They are getting better and will no doubt get even better when (if?) they finish the new airport.