Oh I got it from here but I tracked it down with a Google search.

I can't say if this is the original source but maybe. https://www.theguardian.com/environment/2018/may/21/human-race-just-001-of-all-life-but-has-destroyed-over-80-of-wild-mammals-study

Biomass VS population makes some sense though. Having a million ants would be sure, lots, but having a million elephants would be WTF wholy shit!