I can’t prove it’s not bigfoot.

I mean, its trivial to prove something isn’t Bigfoot on the grounds that Bigfoot Isn’t Real. That’s just Hitchens’s Razor. The burden of proof is on the person presenting the claim, not the one refuting it.

Especially since it’s rather hard to judge scale on airborne things some distance away.

A bunch of the sightings have literally just been stars in the night sky.