Original: When belaying for a partner who is significantly heavier, it is advisable for the belayer to create an anchor point for him - or herself before allowing the climber to lead a route. This will not prevent a fall but will prevent the falling climber from counter-balancing the belayer during a fall and causing a "take off" where the belayer is launched into the air or towards the rock.
Simplified: The belayer can hold the whole weight of the climber by using only a little force, and can even stop a long fall.