I have the same issue. Is there a fix of some sort?

I have the same problem. Before, I played as the Byzantine Empire, all the cities on the Bosphorus were a 6+ slot with no problem and a large population of about 20k. Now I started playing for HRE most big cities end at 6 slot. They do not grow further. Frankfurt and Brunswick, for example (I don't know how to spell it) But I take it. From a historical point of view, this makes sense, as the Bosphorus was more developed than northern Germany, but from a gaming point of view it is very unbalanced. When we realize that there are also such settlements (such as Hamburg), which have ONLY 4 slots. What's more, I don't know if it's a mistake, but Hamburg has ONLY 2000 populations and side Brunswick has over 10k. It would be nice to be able to expand cities by population size as it was in TW Medieval. One slot per 10k population needed? I don't just know the suggestion. I'm mainly interested in the slots and the low population in Hamburg. That sucks. The Nordic kingdom was a dangerous rival. And only 3000 people didn't live there ...