The biological mechanism that explains how obesity increases cancer risk may be different for different cancers. The exact mechanisms are not known for any of the cancers. However, possible mechanisms include alterations in sex hormones (e.g., estrogen, progesterone, and androgens), and insulin and IGF–1 (insulin-like growth factor-1) in obese people that may account for their increased risk for cancers of the breast, endometrium, and colon. Sex-hormone binding globulin, the major carrier protein for certain sex hormones in the plasma, may also be involved in the altered risk for these cancers in obese people.
This answer is based on source information from the National Cancer Institute.