I talked to colleagues from the Dortmund area at lunch break during a meeting about the breweries in Dortmund. They pointed me to a page on Dortmund's homepage, which is as far as I know only availabe in German. There is a a sentence of breweries in Dortmund:

The breweries produce in 1913 1.7 Mio Hectoliter of beer (increased from 1870's from 140000 to 1700000 Hectoliter). And they think Flying Lawyer is right, they have been number 1 at least till late 80ies, not sure, if Dortmund is still German's number 1.