Well the first thing I'd do if a 200lb guy was attacking my a55, is to take out his knees...but I presume that you want to keep sparring with this guy!
There are many components to power. Here's a few things I suggest you can train and then I'll mention some semantics. These are just things that *I* think are good. Like anything try them out for a while, see what they do for you. Moreover...think about what they do more than you practise them. Thinking good.
1. Candle punch: this achieves a whipping power. What you want to do is fire out centre punches one at a time with the bottom two knuckles faced at the flame when fully extended. When you do this punch, make your whole arm loose, and like a whip (and with good form), have the punch come out with your fist loose. Tighten up at the last second at full extension. If done properly, the air will escape out between your knuckles and be forced out and blow the candle out. Obviously, keep moving back as this gets easier. This helps with the whipping power.
2. Sand Bag: Hold a loose, formed fist 1-2 inches from a sand bag. Strike the sand bag with the bottom two knuckles while clenching your fist and immediately relax. Hold the next hand in the same position. Rinse, repeat. This will teach you to strike hard with good form and relax after. Relaxing after each motion is key and will help keep you supple and able to read energies.
3. Spring Jong/Dummy: Place your hand on the Jong(changing the positions as you see fit) with the arm somewhat bent and relaxed. Then strike the jong with this precontacted hand, again - relaxing right after. With Jongs that are mounted with two 1x4's going through them...this works well as the Jong should spring away from you. This teaches Ging, or relaxed (penetrating) striking energy.
These three things are great for developing punching power. If only I did them more often.
Also about generating power:
Where is the hit going? If the hit is to the face and there is little to no gear, use an open palm. When striking his face, don't forget to push along his centreline and this can disrupt the force.
What else is hitting him? Are you uprooting him with a Pak Sao to his elbow/wrist/shoulder/etc? If you especially can push that elbow into his centreline, then this will disrupt power. Your stance is very important. When you strike, also sink your stance and get your power from the ground like a pike or lance with one end in the ground and the other end de-horsing someone.
One last thing: if you can get close enough to have your knee touch his, you can use this to disrupt his power and uproot him. Chi Gerk is good for setting this stuff up. If you are knee to knee, sink and push forward with your back leg (while striking if you like!). The step adds power to any punch as well as the push on the opponents knee is like another contact point in the battle.
Try it, lemme know how it goes.
Best,
Kenton Sefcik
An ounce of action is worth a ton of theory. Friedrich Engels