Peto's paradox https://en.wikipedia.org/wiki/Peto%27s_paradox https://en.wikipedia.org/wiki/Peto%27s_paradox is interesting and relevant to your question. To su

Peto's paradox https://en.wikipedia.org/wiki/Peto%27s_paradox https://en.wikipedia.org/wiki/Peto%27s_paradox is interesting and relevant to your question. To summarize, larger species have evolved better defences against cancer, because they have more cells, so would otherwise experience far more cancer. There's some evidence that these defences reduce fertility, so aren't selected for in smaller species. Also, the evolution of cancer cells to get around an immune system happens on a completely different timescale to the selective pressures for the human immune system to avoid cancer (days vs decades).