cause he is a werewolf so he is hard and Bella is fragile, it also hurt her lebih because well he kissed when he knew she was with Edward, I would punch him too!!
anda have to remember that Jacob was a werewolf sejak that time so he was extremely strong and his build was meant to withstand anything so when Bella punched Jacob there was no effect on Jacob because his Bones were hard to break but to Bella when she punched Jacob it was like punching a wall.