Why does Germany lack a mega city?

Germany is full of old, beatiful, important and big cities but why isn´t there a major city? I mean France has got Paris and the UK has got London. Big and powerful nations tend to have one big city e.g. NYC, Tokyo, London, Paris etc...

I know that Berlin suffered a lot in WW2 and that most of its old architecture was wiped out, however, even before the war Berlin was smaller. Not that Berlin wasn´t important, it was concidered one of the greatest cities in the world, but it has always been smaller than Paris and London.

Germany did not become a nation until 1871 (if my memory serves me well) but still one would imagine that Berlin would have had the time to develop.>