First, I think we should make a distinction between Kaido’s and Big Mom’s durability.
As far as we know, Kaido’s durability is due to his devil fruit rather than an innate ability. Luffy mentions the strength of Kaido’s scale as the biggest obstacle in defeating him.
Big Mom’s durability is likely due to an innate affinity and talent for Armament Haki. The invisible shield that Luffy is currently trying to learn is what surrounds Linlin at all times even at the tender age of five. We see Linlin casually breaks the blade of a Legendary Giant.
Now, the common conception is that her skin is just naturally durable which could very much be true except for one key instance.
Before the wedding invasion by the Straw Hats, we see Capone talking about Linlin and the conditions required to hurt her.
He implies that she can only be hurt when she is emotionally vulnerable. In the same arc, we learn that Haki can’t be used when the individual is under emotional distress.
So, what does it all mean? Linlin may be the most TALENTED Armament Haki user we’ve seen thus far. At the age of 5, she already had Advanced Haki around her at all times giving her the insane durability she is famed for.
Linlin’s Armament Haki capacity is so impressive that she is able to maintain it on her at all the time. Unlike others that are assumed to have impressive Armament Haki, she actually has evidence to support making such a claim.