You said force on the brakes, not the energy converted. the force of friction on the brakes slowing the car down cannot exceed the force of static friction between the tires and the road. once the force of friction on the brakes reaches the maximum static friction of the tires on the road, the tires will begin to slide. since the force of static friction is defined as F=KN where K is the coefficient of static friction between the tires and the road and N= the normal force of the road on the car. since the car is not flying into the air the normal force is equal to the weight of the car. if the two cars have the same tires, then the coefficient of friction is the same. therefore if you increase the weight of the car, the maximum force of static friction will increase. since the force of friction on the brakes has to be equal and opposite to the force of friction, the force will be greater on the car with a greater mass. this is assuming that both cars braking systems are strong enough to lock up the wheels and overcome the static friction of the tires on the road.