Based on our current understanding, probably yes. The largest recorded blue whales weighed around 190,000 kilograms and were 29.9 meters long. Don't forget that. There are two animals that at least some scientists believe may have reached or even exceeded the weight of the blue whale.
The first of these animals is the Lilstock ichthyosaur , whose scientific name has not yet been published. Its length is estimated at 20 to 26 meters, but it was probably significantly thicker than a blue ichthyosaur, so a similar weight is possible.
As far as we know, it wasn't a plankton feeder, but probably not a fish or squid feeder either, since it was toothless. Instead, it likely sucked crustaceans from the seabed, like a gray whale, using benthic suction.
The second "candidate" is the Triassic octopus . The existence of this organism is claimed by one paleontologist, Mark McMenamin, and only by him. The hypothesis is based on rather weak evidence, but it is at least somewhat plausible.
The idea was based on the discovery of 205-million-year-old ichthyosaur bones arranged in a strange pattern and bearing suction cup-like imprints. McMenamin believes they were deliberately arranged that way by some kind of gigantic, intelligent cephalopod. Like I said, flimsy evidence.
Based on the size of the alleged sucker marks, this thing – if it existed – would have been 30 meters long and weighed 200 tons, making it the largest animal of all time. That is, assuming it had the proportions of a modern giant squid.
The Lilstock ichthyosaur is therefore – so far – too poorly understood for us to make an accurate mass estimate, and there is only scant evidence for the existence of the Triassic octopus. Until new evidence emerges, we can say with certainty that the blue whale is the largest animal ever known to science.