The lack of correlation between body mass and cancer risk is known as Peto's Paradox, after epidemiologist Richard Peto of Oxford University in the UK, who noted it in the 1970s. Evolutionary biologists think that it results from larger animals using protective mechanisms that many smaller animals do not.
Evolutionary biologists say that evolution does not always favor tumor-suppressor genes. Although these mechanisms could reduce cancer mortality in any animal, they may come at a cost — reduced fertility, in the case of one research group's model. The result is that for intermediate-sized animals, the evolutionary cost of having many tumor suppressor genes is greater than the benefits of cancer protection they offered. In other words, it is better for the population, evolutionarily, to tolerate more deaths from cancer rather than investing in more costly mechanisms to avoid cancer development. But for massive animals, the cancer avoidance cost of having many tumor suppressor genes is worth it since giant beasts tend to produce few offspring per animal.
Another theory to explain Peto's Paradox is that large animals generate fewer reactive oxygen species resulting from a lower basal metabolic rate and therefore have a lower chance of a suffering a genetic mishap.
Researchers around the world are trying to find the mechanism of Peto's Paradox with hope that it can shed light on cancer prevention and treatment in humans.