Impulsivity is a variable behavioral trait that depends on numerous factors. For example, increasing the absolute magnitude of available choice options promotes far-sighted decisions. We argue that this magnitude effect arises in part from differential exertion of self-control as the perceived importance of the choice increases. First, we demonstrate that frontal executive control areas are engaged for more difficult decisions and that this effect is enhanced for high magnitude rewards. Second, we show that increased hunger, which is associated with lower self-control, reduces the magnitude effect. Third, we tested an intervention designed to increase self-control and show that it interferes with the magnitude effect. Taken together, our findings challenge existing theories about the magnitude effect and suggest that visceral and cognitive factors affecting choice may do so by influencing self-control.