Big mom is quite strong for a kid as we have seen in the latest chapter 866
I think the main reason is because she awakened armament haki when she was a kid
We have seen Aisa and Otohime awaken observation haki by birth
Since observation haki can be unlocked by birth so possibly can be armament haki…
Now big mom has shown killing a bear by just one blow
The reason of this power is not just muscle strength because if it was just muscle strength she could have torn off the arm and fins of those two kids instantly
Just think about it…If someone possesses muscle strength to kill a bear with one blow, she can rip of kids’ arms or fins instantly before even they could ask for help (even though she was not able to tear them off even after that)…So it must be armament haki as armament haki can increase the power of a blow, it cannot increase the brute/muscle strength of a person…Thus the blows becoming powerful but pulling had no effect…
This also explains her power for defeating all those giants as most of those giants probably knew no haki…
I don’t by any means mean to say that she had no muscle strength…What i meant to say was that with huge physical strength she had awakened armament haki by birth and use it without even realizing…or else she could not have defeated all those giants with just physical strength…
During her tantrum while a child she probably used all her muscle strength (as we all know human beings are limited to the use of muscles and cannot use the full strength of muscles) and huge amounts of haki…
*Theory by Mugiwara D Ace