We can't say with certainty that the blue whale is the largest animal ever, but to our knowledge at most there were several other species approximately the same

We can't say with certainty that the blue whale is the largest animal ever, but to our knowledge at most there were several other species approximately the same size. And even those are based on estimates with rather huge ranges. https://en.wikipedia.org/wiki/Largest_and_heaviest_animals https://en.wikipedia.org/wiki/Largest_and_heaviest_animals