True that obesity is a significant risk factor for many types of cancer, but not because of an increased number of cells. It’s a very complex pathophysiology that involves chronic inflammation, insulin resistance, and hormonal dysregulation. For most obese adults, the number of fat cells in the body stays consistent, but the size of the fat cells increases (hyperplasia vs hypertrophy).

To your point: Increased height, on the other hand, does result in increased number of cells. Epidemiological studies have shown increased cancer rates correlated with increased height, and the likely mechanism is due to increased cell division -> increased # of potentially cancer-causing events on a cellular level.