spraying inside the litter box?

I just bought a new litter box and put it down with fresh litter. Neo jumped in and I could hear him going (hear it hitting the side of the box. It is a good thing I used inclosed litter pans!! Is there anyway to stop this? is it because neo is so big and the new little pan isn't the extra large kind?? Or is he just being good and spraying because of the kittens but doing it in the litter box!

It's a new box and he is marking it as his!

Or if he went to the vet he was upset and spraying his displeasure.