Couple things.
1) wasn’t muscular already weakened from another student?
2) Deku did a controlled 5%? I could’ve sworn he like put all his power into that blow against the Nomu since he literally just saw Asui about to be murdered right in front of him. Shigaraki even mentions how Deku was about to “murder him” when he did it.
3)
This page looks pretty confusing to be honest but it looks to be Shigaraki is slamming Bakugou’s chest in. (His heart being pierced wasn’t from Shigaraki, it was from his own explosions going on internally inside his body which made his heart pop.)