Actually, it makes sense to me that influence, money, and tourism would scale linearly with population, but not production.
Economies scale far better than linearly with added population, because you get exponential possible transactions. Thus, with 2 people, there's really only 1 transaction relationship possible. With 3 people, there's 3 possible transactions (A-B, B-C, C-A). With 4, you get 6 (A-B, B-C, C-D, A-C, A-D, B-D). And so forth. It's O(n^2).
Naturally, that's optimal, and never happens. But an economy does show far greater than linear scaling as relates to population.
Influence, on the other hand, depending on how we define what is going on with that metric, doesn't have to scale with population at all.
And Tourism similarly doesn't depend much on population - indeed, it seems to be more related to land area than anything else I can find, though even that is shaky.
(I just realized this is ambiguous. I'm talking about real-life Earth, not the GC3 universe.)