In all kicks, a specific area of the foot is the striking surface. To accomplish this, the foot must be held in a specified shape at the moment of impact.
The supporting leg must be slightly bent and springy, with the foot firmly on the ground. Having the knee too straight or too bent will adversely affect the kick. The supporting leg must be supple rather than rigid. If the support knee is locked, the leg is susceptible to injury, either internally from the force of the muscles, or externally from a possible strike to the leg. Remember, any force you apply to the target is also reflected back into your body. When the knee is unlocked, it also permits subtle leg movements that allow the kicker to maintain balance.
Do not raise the support heel in an attempt to gain more height in the kick. With the heel off the floor, power is lessened since the springing action of the ankle absorbs forces being transmitted to and from the floor through the body.
The lighter your body weight, the more speed is required for you to develop a kick of a force equal to the force generated by a heavier student. The actual velocity of a kick is, perhaps more than anything else, determined by the power expended in the snap of the knee.