It's not consistent across species because different species have different mechanisms for averting tumor growth. But within species (certainly among humans, and I'm pretty sure also among others), more cells means more opportunities for something to go wrong and so more cancer.

Counterpoint: elephants.

Do larger elephants have less cancer then smaller elephants? If not, that's not a counterpoint.

Isn't that because the ocean water blocks radiation?

as one cause of cancer. there are many others.