You can see in this video how quickly the chicks have been growing. If you look very closely you can see that the older nestling is just starting to open its eyes!
According to growth charts for catbirds from the Cornell Lab of Ornithology Birds of the World (https://birdsoftheworld.org/bow/home), nestlings are about 3 grams when they hatch (a bit more than a penny). But by day 4 they should be close to 13 grams (a bit more than a AAA battery)! So those little meals really add up, and at this point the mother and the father will be starting to spend less time brooding and more time feeding, and by day 5 they should be feeding the chicks roughly equally.