Agree about scale making qualitative differences in events, but CCC has been happy operating at larger scale organizationally. I disagree with a lot of the organizational politics with CCC, but they're amazingly good at organizing events, and those events are really good. Combination of the main org/structure, volunteers, and especially making it somewhat federated with other groups/chapters doing their own stuff during the event as well.

In this case CCH doesn't really have more space, and Leipzig sucked as a city to host events (no hotels/food/etc. immediately near the center, etc.). CCC is not so profitable as to get the truly top-end convention centers, and cares about the total cost/experience to participants (accommodations, food, side events, travel logistics). Hannover, Frankfurt, Dusseldorf, and Munich have the biggest ones in Germany. Berlin is almost as big; I think it's really expensive to book but if I had the money that would probably be much choice for ideal CCC location. CCH is really quite small as congress centers go.