Much stronger bomb, she also took this while she was already on her last legs. Even this bomb blew a hole on the bottom of Onigashima and destroyed Kanjuro’s fire painting which Yamato struggled to put out. A healthy Big Mom took no damage from Pedro’s explosion.
I'm talking about Big Mom raw durability, like Zoro against King she can likely defend herself from explosion with haki in normal circumstances but here she was too weakened for that.
You misinterpreted the scene in this chapter imo
The bomb intent was never to kill Wb & co, it was to trap them underground by destroying the exit/entrance (which is why WB used the gura to make an exit), it just happens that the bomb fell next to Shiki's rival (who was in front of WB/Kaido, closer to the exit) and it exploded near him cause they have a funny relationship. If the explosion directly hit Kaido or WB I doubt they would've been so casual, shrugged it off or not. They aren't the type to let others disrespect them, especially Kaido who is known for his violence, and his temper seemed even worse back then.
Anyway I don't know if Silver Axe is on the level of the future yonko (being a rival does not mean exact same level) but Shiki certainly got nothing to envy to them in portrayal.