There are tons of things in Zalanthas, the physical world, that don't make obvious sense or aren't really explained in the docs: Where do plants get their water from? Why are the insects so big? Is Suk-Krath really red, and wouldn't that mean that it should look huge, given that the planet is in the livable zone of it? Where could the Black Moon have come from, really? How do Mekillots actually get enough biomass to eat?
This thread is intended to explore such questions in a completely hypothetical manner, with healthy doses of scientific support where it's needed. I don't imagine this should affect things in-game, where it's often better explained with "magic" anyway, but for those who need it, it might give plausible deniability to these things.
I propose that Zalanthas has an atmosphere of almost 100% oxygen, but less gravity and therefore lower air pressure. This would support giant insects, as Earth has produced at least 70 cm long ones back when we had more than 30% oxygen in the atmosphere. This would be toxic for humans, however, at Earth's air pressure, so we would want pressure to be lower (equal to about 4000-6000 m altitude on Earth). This lessens the ability of insects to absorb oxygen (they don't breathe*, but rather absorb it through lots of tiny tubes called trachea), but might help them grow as gravity does not put as much of a strain on their bodily fluids. The same goes for our vertebrates - less power needed to supply oxygenated blood to brain means larger size is possible.
Potential issues with this theory: fire. Things would burn, a lot.
It might be standard gravity, but about 60% oxygen instead - just barely survivable for humans (any more and our lungs get inflamed and kills us).
* Technically, I read somewhere on the Internet that they breathe a little, but that it's negligible.