Hi, after reading a lot on Bruce Lee, for some one who just under 7 and a half stones, he was known to have one of the hardest punches in both hands. his secret to this was he put his heart in to it (ie grew balls to fight)
However you can also to a form of training called isometric training at home, its simple.eg put a pillow against a wall and in a punching state punch the pillow and push against the pillow for about 15 secs keeping the arm straight, then do the same bending the arm approx 45 degrees, then do it again bending the arm fully. so after the third time you would have completed a full punch, its best if you read up as i cant explain it properly, hope this helps