How did mites get so small?
We don’t know much about the early evolutionary history of mites – or the earliest arachnids in general. A few mite fossils are known from the Early Devonian (~410 mya), and there’s a possible Ordovician specimen, but they’re all already tiny.
They probably got so small simply because it provided some sort of evolutionary advantage to them. Since they seem to have originated as terrestrial animals at a time when complex plants and ecosystems were only just getting started on land, with limited food resources and habitats available, reduced size might have allowed mites to exploit niches that other arthropods weren’t already occupying.
And it obviously worked out really well for them, and they never needed to get particularly large ever again, because they’re now ridiculously diverse and are found pretty much everywhere on the planet.