A few animals spring to mind- the Tapir, some antelopes are small-ish, and there is a breed of wild cat that is about fifty pounds or so. Perhaps it’s because small animals can hide, big ones intimidate but those in the middle need more specialized evolutionary traits to help them survive and thus, it knocked out a lot of the ‘misfires’ over time. Like an octopus- how many failed variations before we got that miraculous creature?