I was always a pretty thin guy so most of the people I trained with were much bigger than I was. The answer is always fast movement. Start off just outside their reach. You can fake a step in to see how he would react if you come at him fast. This will help you know how you should come in at him when the time is right (e.g. weaving low under his cross or parrying the jab/teep then counter how you like). But lets go back to the outside where you started. Most likely the guy will lead with kicks. Most people do and people with longer range (like big guys) do it more. You have 2 usefull tactics here: 1) step backout to evade the kick (parrying the teep hels too) and then come in hard and fast as they are retracting to hit them, and move out another direction. 2) Step inside the kick, catch the leg, lift it and kick the support leg out from under them. Big guys get really humbled after you have taken them to the ground a few times. When kicking the support leg out, you can soften the kick (to avoid hurting them) just by pushing on the back of their knee. They will go down just the same. This is how you can fight the bigger guy at long range.
If they want to come in with punches (likely to happen after you have employed the above tactics a few times), then you aim your low kicks to the leg that they are stepping on to e.g. (assuming orthodox) if they step forward on their left foot and jab, then you throw a sharp low thai kick (no switch) into the inner thigh of their left leg just before it contacts the ground. This will disrupt their forward momentum and change the angles of their incoming punches. You can then defend with your high guard, counter them whilst they are trying to get their balance back and then move out in another direction.
Being big and strong is a very good tool for a fighter but it isn't the only ones. Be fast- they can't hit what they can't catch. Be smart- draw their attention high to hit low and vice-versa. Have faith in yourself- you can't expect to win by hitting them from a safe distance. Don't let them grab/clench you- this will nullify most of your speed/movement advantage. Always look for ways to disrupt their balance- makes it easier to take them down and weakens the power behind their attacks. Don't play their game- you will come off second best if you try trading blow for blow in the pocket with a bigger stronger guy. If they are fast, slow them down. If they are strong, be too fast for them. If they like long range, you fight at short range. If they like short range, you go long range. If they move alot, trap them in the corner. etc In essence, there are many ways to fight against a bigger opponent. You just have to use ALL the tools at your disposal.
