The incidence of many kinds of cancer are significantly higher in obese people than others. The important question of course is whether obesity actually causes cancer or whether the two illnesses are linked to some third and perhaps hidden agent like genetics or exposures say to a virus that might somehow trigger both obesity and cancer. The evidence is growing that obesity itself is to blame.
Cancer cells can be grown in a petri dish provided they are supplied with certain key nutrients and, interestingly, provided that the hormone insulin is added to their growth medium (the "broth" that they are grown in). If insulin is NOT added to the growth medium, the cells will not grow. If it IS added, they proliferate rapidly. This has been known for decades and it has long aroused in researchers a suspicion that insulin could, in some manner, be a tumor-promoter.
The link between insulin and obesity is metabolic syndrome via insulin resistance. It probably "works" something like this: obesity causes metabolic syndrome, metabolic syndrome causes insulin resistance, insulin resistance causes the pancreas to "push" blood insulin levels higher than normal in an attempt to overcome resistance and this elevated blood insulin level promotes cancer. Is this mechanism proven? Absolutely not. Is it possible? Absolutely. Is it likely to be a mechanism that explains the proven elevated cancer risk of obesity? I think so. I may be wrong. Time will tell.