As Koloss grow, their skin goes from super loose to stretching and breaking.
They are also fairly violent creatures who fight a lot. Which, of course, damages the skin more.
But Koloss also reuse the skin when creating new Koloss.
(all bolded emphasise mine)
So if the skin is reused, but also extremely prone to damage, where does new skin come from when they need to replace one? Over here, someone postulates that it is the skin of the "host" human, but I'm not convinced.