Why is this cat coffin so small?
This coffin likely only contained a partial cat mummy, parts that symbolically represented the whole.
Cats were one of the types of animals that were both beloved pets and sacred to the gods. They symbolized both domestic, maternal qualities and could stand in for the powerful lion. Cats were also venerated for their ability to see in the dark and catch vermin that threatened the home.
Would this have just held ashes?
This actually would have head a very small cat's body (a kitten or a fetus) which would have been made even smaller through the mummification process.
What was this cat coffin used for? It’s too small for a cat mummy, no?
You're absolutely right, that coffin is definitely too small for a full cat. Likely parts of the animal would have been included in this coffin instead of an entire cat. You can see the small portion in the side of the coffin where it opens.