Originally Posted by Qwertty
work on getting your knee(on the striking leg) up higher. Going up on your toes and twisting your hips a bit more will help with that. Your shin should be traveling downward or at least across when it hits, not up. And with your right hand, try reaching out in front of you and "pull" your body towards the bag, kinda like your grabbing the air and pulling it. This will also protect your face while you start the kick
Good description Qwerty, the only other thing I can add is when the kick ends e.g hits the target, your front foot left in this case, should be facing almost behind you as pivot left on the ball of your foot and turn your hips in same direction, this should be 1 fluid motion through the target, instead of hitting it and pulling away when you do so. As your quite a big fella you should be able to do some good damage when you learn the right technique.
The only other thing is you'll know when it's about right as you'll notice more of a quiet thud more than a loud slap when the kick hits the pads.
Good examples are Buakaw por pramuk, Gilbert Yvel, Mirko Cro Cop to name a few.