Hi! I have twice gotten a random event that my pet's treasure chest has grown...but when I go to my pet page, it shows no treasures. Am I looking in the wrong place?

the event is referring to how many slots you have in your treasure. For instance when it grows by 2, you can add 2 more items.

Hover on the personal dropdown, then hover on pets, under your pet's name choose "pet" then on that page choose "Edit Treasure" and you can see how many treasure slots you have.

When you have an item you like, you can select to put in pet's treasure.

perfect thanks!