Why are Midoriya's arms weaker than his legs and why do they get broken more easily?

Question

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?

Answer 1

From the start of season 1, Midoriya always used his arms, but since he is not able to control power output that caused his bones to shatter all the time. With the help of Gran Torino he was able to develop Full Cowl, but that was not enough. His fight with Muscular was a prime example of that. Thus he again shattered his bones multiple times in that fight, and that was the "wait stop" point for him.

The human body can fix fractures like that to an extent, but he was using this again and again. So much that even Recovery Girl said "I can't fix you next time."

Therefore, if he continues to use his arms with uncontrollable power output, they will break to the point of him not being able to use his arms. With the help of Full Cowl, Midoriya can use 20% of his power now, but that's a recent power up he got. To not risk his arms, he decided to find another solution or fighting style. Using his legs was the answer.

In conclusion his arms weaker than his legs because he shattered his arms so many times.

Answer 2

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.

Answer 3

You are missing important thing - Mydoria had never used 100% attacks with his legs.

Mydoria was breaking his arms only when using 100% attacks, and since he was not aware of other ways to use his power early in story, it was happening a lot. Eventually this led to serious damage on his arms, up to the point when any further usage of 100% attacks would cause permanent damage.

This is why he has started to think about alternative ways to fight, and ended up using his legs. But with legs, he was using Full Cowl and iron soles from very beggining.

Therefore, I think it is not fair to say, that his arms are weaker than his legs, as he started to use those in completely different circumstances.

Update: actually, good example of his legs being as fragile as his arms is his academy entry exam - when he used his legs to jump, they've broke just as his arm he used to hit the enemy.