Cancer isn’t directly tied to the nu,ber of cells. It’s the type. Cancer shows up in places where cells replaced most often. Some cells are long lived, others not so much. The more replication you have, the greater the chance of a mutation that ends up being classified as cancer. So while a whale is huge, a lot of that size comes from things like muscle and fat, which don’t replicate once mature (I believe). So the complexity of the human body causes it to have more things that need to replicate, and thus more opportunities for cancer. And then of course, plenty of otther things we do that bump up our chances…