Izuku managed to do a proper smash this time, so his arm didn't break.
"Proper smash" is all about control (and/or endurance). He screwed up his fingers last episode when he threw everything into conserving himself and it didn't work.

Five minutes later he freaked out and only thought about smashing fuckface to save Tsuyu, and somehow that got his control right? I don't think so.

I also think it's about training his body. It's not about strengthening in the physical sense, but having it get used to its effects (whatever the hell that means). If it's just about "controlling", then that means One for All can only be weaker than Izuku's "all or nothing" punch. That's sad.