I'll disagree with Brian on this one. If you want to develop your grappling skills to the level where you can control and defend yourself from a bigger, stronger opponent, then free grappling without strikes is an excellent training method. The point isn't to prove how tough your are - it's to learn. Without the distractions of punches coming at your face you can pay a lot more attention to studying the subtleties of balance, structure, and energy that are necessary to overcome someone who has superior athletic attributes.
Learning to overcome a bigger stronger opponent in a free-form setting where he is free to use his attributes as he wishes takes a lot of time and practice. It can be frustrating - you'll find times where you have superior leverage but his strength is sufficient to overcome your leverage advantage, which hardly seems fair. You need to go through the experience enough times to learn how to never give them the chance to use their full strength against you. Be patient and keep working at it. There are no shortcuts.
Once you're pinned on the ground I'm not sure that Tai Chi has a lot to help you. You may have to look at some of the grappling arts which deal with that situation.
Learning to overcome a bigger stronger opponent in a free-form setting where he is free to use his attributes as he wishes takes a lot of time and practice. It can be frustrating - you'll find times where you have superior leverage but his strength is sufficient to overcome your leverage advantage, which hardly seems fair. You need to go through the experience enough times to learn how to never give them the chance to use their full strength against you. Be patient and keep working at it. There are no shortcuts.
Once you're pinned on the ground I'm not sure that Tai Chi has a lot to help you. You may have to look at some of the grappling arts which deal with that situation.