Most people who overeat also tend to eat many sugar-laden foods, which promotes elevated blood sugar levels and insulin resistance. So overeating sugary foods equates to a double-whammy in terms of cancer risk, compared to overeating whole, unprocessed fare.
Cancer cells are primarily fueled by the burning of sugar anaerobically. Without sugar, most cancer cells simply lack the metabolic flexibility to survive.
Normal, healthy cells have the metabolic flexibility to adapt from using glucose to using ketone bodies from dietary fats. Most cancer cells lack this ability so when you reduce net carbs (total carbs minus fiber), you effectively starve the cancer. This is why nutritional ketosis appears to be so effective against cancer.