Did very tiny animals, particularly insects, evolve to be smaller or did they start smaller and got larger?

Fleas, tiny gnats, spring tails, mites, lice, are all examples of small animals.

But, are they typically shrinking in size over evolutionary time, or getting larger. I don’t expect all of them to necessarily have the same evolutionary history, but I am interested in the pressures that might have shrunk them to such small scale.