TFA: "Today's tiniest sea critter is less than 10 times smaller than its Cambrian counterpart, measured in terms of volume; both are minuscule crustaceans."
Presumably we're talking about Stygotantulus stocki, which is 94 µm long. That's already smaller than a lot of bacteria, so I'm not sure how much more it could possibly shrink? There is an effective lower bound here.
TFA: "But at the other end of the scale, the mighty blue whale is more than 100,000 times the size of the largest animal the Cambrian could offer: another crustacean with a clam-like, hinged shell."
Leaving aside the apples-to-rutabagas comparison of clams and mammals, is there any physical reason why whales are the largest animal we could imagine? They're big, but so is the ocean. There is no effective upper bound in sight.
Average is not a meaningful statistic in this case.