Another thought: what about having larger prehistoric bugs in Savage biomes? Griffinflies, giant millipedes, etc. It might break the realism a bit, but it's not like they didn't use to exist, and it would be a cool way to keep Savage biomes, well, Savage.
https://en.m.wikipedia.org/wiki/List_of_prehistoric_insects
Wouldn't those carboniferous arthropods be more like megabeasts in size, than savage creatures?
You're not wrong, but let's examine it for a second.
According to Wikipedia, black ants (our main playable race) are about 5mm long. Queens and soldiers are bigger, drones are smaller, but the workers are about 5mm, so lets use that for right now.
Arthropleura armata (Which I would just call a giant millipede to avoid using scientific names) was around 2.5 meters, or 2,500mm. Since brolol.404 stated earlier on that black ants have a size of 100,000 in game (unless I misread), we can use a simple ratio to find that (100,000*2,500)/5=50,000,000, or about 25% larger than a giant elephant.
A. armata was an herbivore, so it would basically be analogous to a particularly large giant elephant, peacefully meandering through the map, largely ignoring our ants unless provoked.
We can use the same process to find the size of a griffinfly. Griffinflies had a wingspan of around 70cm, or 700mm. (100,000*700)/5=14,000,000, or quite a bit smaller than a dragon. They would be horrific predators, but also quite rare.
Most prehistoric insects weren't all that big.
Ororaphidia, an extinct genus of snakefly, was only 12mm long, or about 240,000 in game.
Afropone, an extinct genus of ant, was of a similar size. I'm sure there are plenty of extinct bugs that were quite a bit larger, but most seem to be around the size of what we call a really big bug today, not the horrific giants you usually see on TV. You could just give them a more generic name and treat the like regular giant animals.
Also, yes, I know using a simple ratio might not be the most accurate way to calculate the size of living organisms, but vanilla DF already kicks physics in the head when it comes to animal sizes anyway, so who cares? R.I.P, square-cube law lol.