The key phrase in your problem statement is "more value", which assumes that all value can be projected onto a single axis. Typically [1], this means that you can assign a dollar value to any change in the physical world [2].
The "debate" for many people is whether this simplistic economic model can be used at all. They say that value is multi-dimensional, and those value dimensions are not fungible. Concretely, if making a medicine factory results in the extinction of a specie, you can't say that it is okay, because the benefit of the former is incomparable to the loss of the latter.
Coming to population, more humans mean more destruction of ecosystems. And it is not clear whether destroying ecosystems can be fungibly compared to the additional human economic value those extra humans create. It's not about more or less, it the incomparability of the two.
[1] but not necessarily what you meant
[2] Just to be clear, the economics discipline already has a wealth of research that humans cannot in fact consistently assign absolute or relative dollar values to the same thing. Yet our political-economy and popular conceptions have this as a central assumption.