I originally thought that the large size dish was too big for my guinea pigs, and had intended to exchange the larger size for a smaller size, however, I found it was easier for our guinea pigs to step in and look for the foods they liked. I decided to keep the larger dishes.
If your cage is a larger cage, you may want to get the larger dish. I've had a cockatiel before and remember how much he loved to step into his food dish and search around for his favorite seeds and fruits. If your Quaker Parrot loves to step into his/her dishes while he/she eats, you may like the larger dish. If you are using the dish for water, it may be best to buy the smaller dish.