Aikido is bad. Almost all styles of kung fu are bad (with the one exception being next to useless for your purposes). JKD is almost always bad.
Kyokushin will teach you to kick people's asses. Yes, even if they're bigger than you. Yes, even if you have bad knees. Yes, even if it's on t3h str33t. Judo will teach you to throw people on their heads. Yes, even if they're bigger than you. Yes, even if you have bad knees. Yes, even if it's on t3h str33t.
As for people saying that small guys can't punch hard, here's a list of the world record holders for punching power by weight.
Heavyweight (200+lbs) - 1081 PSI
Light Heavyweight - 1023 PSI
Middleweight - 987 PSI
Welterweight - 951 PSI
Lightweight - 923 PSI
Featherweight - 909 PSI
Bantamweight - 843 PSI
Flyweight - 829 PSI
Strawweight (105lbs) - 813 PSI
Note that the strawweight was able to punch with ~75% of the power of the guy that weighed at least 2 times as much as him. It's all about technique.