Why are Midoriya's arms weaker than his legs and why do they get broken more easily? In episode 14 season 3 of My Hero Academia we've seen that


   Midoriya develops an attack with his legs because his arms get broken more easily when he uses One for All


Why are Midoriya's arms weaker than his legs and get broken more easily?
Human legs are naturally stronger than arms. This is something I already knew, but to back up this claim I checked wikipedia: 


  As the human leg is longer and stronger than the arm...


While I was watching the show, I was given the impression this was the reason he switched to Shoot Style, not because of his arms being repeatedly broken and thus weakened. The My Hero Academia Wiki backs up my memory (emphasis mine):


  To use Shoot Style, Izuku concentrates Full Cowl's effects in his legs rather than his arms for his smash attacks, enabling him to use their naturally greater power to improve his striking strength.