You know, Kaptayn, when Rob Lucci revealed his ability for the first time, he said that zoans' special quality is to 'strengthen themselves physically'.
http://i.imgur.com/uaNo3LG.png
And he further added to that that their powers increase the more they train.
http://i.imgur.com/KpmFbEZ.png
This kinda reminds me of the Saiyans: they grow stronger than they were previously after their body recuperates from the damage received from a near-fatal injury(ies).
Could that be how Kaido became this sturdy? He fought and fought and survived each and every one of his fights until he became the monster he is, & all was due to both his devil fruit and his skill in battle.