There could very well have been something bigger but never left fossils. I mean fossil creation itself is sort of a miracle. Combined with such an animal really bound to living in the ocean, it would be very difficult for a whale the size of a blue whale to
1: meet the conditions to be fossillized
2: survive millions of years
3: discovered at the bottom of the ocean
An astronomer, a physicist and a mathematician are on a train in Scotland. The astronomer looks out of the window, sees a black sheep standing in a field, and remarks, "How odd. Scottish sheep are black." "No, no, no!" says the physicist. "Only some Scottish sheep are black." The mathematician rolls his eyes at his companions' muddled thinking and says, "In Scotland, there is at least one field, containing at least one sheep, at least one side of which appears black from here."
On top of that, oceanic crust is all quite new, geologically speaking. Even if something large and oceanic did manage to fossilize, unless it was also in a shallow interior sea, whatever rock it fossilized in has been subducted and destroyed past a few hundred million years.
it's not unfounded. there's direct supporting evidence. and they're not really absolute. it's blatantly obvious that the "that we know about" is implicit.
There could very well have been something bigger but never left fossils. I mean fossil creation itself is sort of a miracle. Combined with such an animal really bound to living in the ocean, it would be very difficult for a whale the size of a blue whale to
1: meet the conditions to be fossillized 2: survive millions of years 3: discovered at the bottom of the ocean