Why is human population increasing? Better kamma? Human population is increasing whether we like it or not and might even hit 10bn in the next few decades. But why is this happening according to a Buddhist's perspective? Some that I know that don't believe in kamma use this as a counter example saying that if people are becoming more sinful they ask how come the population is increasing. While I have my own understanding (devas being born back among humans, kamma alone doesn't determine the next birth "cuti sitha" does) I would like your expert opinion this.
Here is not an expert opinion: which is that I don't know why you expect that the total number of sentient beings is finite, countable, and constant.

The perspective in the OP seems to be that the total number is constant, and is asking, "If the number of humans is increasing, then where (which realms) are they (pre-existing beings) coming from and why?"

It has a view of, "this used to be (countably) one old sentient being, and that (same) being is now being reborn as (countably) one new sentient being" ... it has a view that, the type of being and/or realm might change (hell, animal, human, deva), but the total number of beings must be constant.

I don't see why you'd expect that view to be true, though: it seems to me an atman (self/soul) view, not an anatman view.

So far as I know a more anatman view might be look "there's no person now" (there are are just skandhas, an assemblage of parts, c.f. the parable of the Milinda's chariot), "the number of sentient beings isn't countable" (except perhaps to the Buddha), and fabricated things are inconstant.