But the overall vibe of the event definitely changed for the worse (imho) with the move to Leipzig simply because the venue there is mostly focused on trade fairs and thus just so much more wide-open with everything much further apart. In Hamburg you'd constantly run across people you know in the hallways due to the compact horizontal layout, and the overall atmosphere was just a bit cozier. It was still a great event in Leizpig, but for me at least it really didn't compare to the previous iterations in Hamburg.
It's definitely a difficult trade-off to make and I'm glad I'm not the one making these decisions.
I don't know how much of that is possible at the CCH
What doesn't scale is the people who organise everything. If they don't want to organise ever bigger events, then they are free to switch back to a smaller scale. It's not only the organisational overhead that increases, but also that a bigger event is not just a bigger event, but also becomes another type of event.
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.