Biggest Cities once in the German Empire

One could plausibly argue that the Frankish Empire was German, or that the HRE was. What about the Hapsburg Monarchy? None of these really matter if you are only looking at a specific state, but drawing from both the Second and Third Reichs really implies you're going to include the First as well, unless you explicitly say you aren't. Then you run into the problem that the First Reich could be defined as all three states I mentioned, because to some degree they were (very) separate phases of the same overall entity.

As for the occupied territory, your source is showing the "territory at its greatest extent", which is decided *not* "all territory occupied". As hwes pointed out, for example, as the Italian state collapsed, Germany moved in to occupy much of northern Italy, but by that time they had lost huge parts of the east so their overall territory was much smaller. This was also true for (some) other Axis members, although that probably wouldn't matter for this quiz.

The current description, with a link to the Wikipedia map of the Third Reich, is helpful. However, it makes it seem that that is the only territory under consideration. You should make a similar link to the German colonial empire, since that is also territory that is under consideration here.

Perhaps this is the link to use: https://en.wikipedia.org/wiki/German_colonial_empire