A small caveat. Largest that we know of. Most animals don't make good fossils, it's actually rare for an animal to get in the right spot at the right time to fossilize, and then we have to find it. Which effectively mean that it has to be on land, preferably empty area, close to surface.
This effectively means that we mostly get fossils from species that very relatively abundant, lived in what is today North America, Australia, Europe, China or more recently, richer parts of South America. Because finding fossils without funding is difficult.
Well, we don't just base the hypothesis that blue whales are the largest animal even on their size relative to estimated size of extinct species in general. It's based on the phylogeny of quite a few extinct and extant whale species and other allometric and paleo-bio/ecological considerations that go into a model of a recent diversification of larger whales that has thus far culminated in the blue whale [1]. It doesn't appear individual species of whale were evolving gigantism really rapidly and out of nowhere from smaller species, but a gradual trend over tens of millions of years. It appears missing fossils that are extreme size outliers probably become less likely the further you go back in time from the past few million years.
> Because finding fossils without funding is difficult
Eh, not really. They don't occur in isolation typically, a formation/deposit or Lagerstätten [2] rich in fossils are accidentally discovered, word gets out at some point, and then international funding follows.
IIRC the blue whale is larger than all possible (reasonable) land animals, so the only possible competition would be another ocean animal of some sort.
This effectively means that we mostly get fossils from species that very relatively abundant, lived in what is today North America, Australia, Europe, China or more recently, richer parts of South America. Because finding fossils without funding is difficult.
There are obviously exceptions.