The biggest one (aside from obvious examples like sponges and jellyfish) has got to be dragonflies. The original dragonfly was as big three-dimensionally as a footlong sub from Subway, though it was a breathing machine (like us, so let this be a true fable for us to learn from) as it needed oxygen that wasn’t in large enough supply. So it simply shrunk in size to adapt once all the air ran out and there was no Druidia to restore it, and ever since then, we’ve had the same tiny dragonfly model ever since, for almost a billion years.