I bet decent money its logarithmic.
1000-9999 = x, 100000-99999=x/2 100000-999999=x/3 etc
My line of (completely untested and unproven) thought is that it'll be a logarithmic effect, but one which, at some point dependent on the population, starts delivering exponentially less influence for missions. Not zero, just less (an S-curve). The two reasons I'm thinking along these lines is:
- (very anecdotally) my observation of 2% change for high influence systems actually came from looking at a system of pop 400-500m, not my 1400
- My understanding is the population based "cap" on change is designed to make big changes to big systems hard (probably so FD can stave off things like Sol becoming an anarchy), erring on small changes over a long period instead.
But a lot more testing needed.