big weight difference. 100lb is a lot in fighting. your punches will not be very significant to his body since he weighs alot more than you. what i recommend is you need to kick, not punch. go for his legs and do leg kicks. kick his leg and pull out, and do that multiple times to make him feel pain on his legs. you should not go f or his body. dont hit his neck or face, unless it is really necessary since he can counter you because arms are shorter than legs. you should watch out for a take down. he can charge you hard and slam you on the ground and god know what he will do. he can kick your face or throw punches here and there and you are done due to the size difference. street fight can be very different from UFC. use your elbow too if you can.