Why do tu think Nuka's mane never grew bigger?
I don't think it was an animación mistake because they totally remembered to develop Kovu's mane and Vitani's size and all, so I don't see any logic reason why they would forget about Nuka. Especially as he was to have such an essential part to play as dying.
So, why didn't Nuka's main grow bigger in all those four to five years that Zira trained Kovu? What do tu think?