You're not really getting the point
Big mom's skin, as durable as it is, it's still just skin
It can stretch, kid can apply enough pressure to reach her inner organs
Think of it like this
Kidd's metal pushes big mom's skin, big mom's skin pushes her body, her body fat then pushes her organs
Also uojrr you're forgetting a big mom detail, kidd's magnetism is ridiculously powerful, so powerful in fact he can keep the metal from tearing apart