How do you get more space for your pet's treasure?

you use the magical keepsake box or MKB (can be bought from the millionaire center) cost 150k

There are several ways. Sometimes you can gain spaces through random events, and I'm pretty sure you can get spaces from the zapper too. You can also buy from the Millionaire Center or from the Cash Shop, which will increase your treasure spaces by a random amount.

Thanks!