Correct me if I'm wrong but , as far as I know is that the stiffer the spring is , the lighter the rollers must be to compensate against the friction between the belt and the pulleys.
The springs only fuction is the keep the belt from slipping between the pulleys. The stiffer the spring , the more friction there is , and the more grip there is between the belt and pulleys. Grip = friction , means less HP.
The more friction there is between the belt en pulleys , the lesser the HP will be. That's why you need the right spring. Just strong enough to keep the belt from slipping , and just soft enough to have the most of power.
To compensate against the friction between the belt and the pulleys , you will need lighter rollers. If the rollers are to light , your rpm will be to high , and the belt might slip anyway. To heavy rollers , and your rpm will be to low , and you might not have enough power to compensate against the tougher spring , and accelerate.