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

Insects used to be a lot bigger in prehistoric times, because oxygen content was much higher than today, so atmospheric composition puts a limit on their size.
This also holds true for other animals, but for insects especially, since almost all of them have a passive respiratory system, which relies on passive diffusion of air into the body, which makes it harder to supply oxygen to all parts of the body with larger body sizes.

@ragingloli Beat me to it. If you look at insect fossils they were often huge. It is a direct result of the oxygen levels in our atmosphere. Insects basically absorb oxygen and this is why dish soap and water is so effective in killing them. There are also some limitations due to the ratio of gravity and density of our atmosphere on flying insects.

So, any little mite that fits on this little dot . <— probably had a big arthropod ancestor, right?

Yep. A dinosaur today would require oxygen tanks. So much for Jurassic Park, and dragonflies with 2 foot wingspans.

Both!

So with insects and arthropods (things with exoskeletons) in particular, there was a period in Earth’s past that had intense oxygen levels where they grew HUGE. Not like ‘monster movie’ large, but we’re talking centipedes that are several feet long and such.

Evolution through natural selection doesn’t just go one way. It doesn’t have an ‘end goal’. It’s always about selecting traits that are best for the CURRENT environment the population finds itself in. When that environment changes, the selection pressures change.

Island Giantism and Dwarfism are extreme versions of this. Look up the youtube channel Atlas Pro for it’s videos on that subject. They’re excellent.

Generally speaking though, there’s a problem with being exo-skeleton based. These animals don’t have the same kind of lungs and circulatory systems we do. Oxygen has to come in and migrate to the center of the organism, and the less Oxygen rich the atmosphere is, the harder that is to do…so insects have to stay smaller. A ‘growth ray’ like in a sci fi movie to make a huge ant…would result in a DEAD ANT very quickly because even if it didn’t just collapse under it’s own weight, it would suffocate at that size.