Birds became itty-bitty by truncating the juvenile growth spurt that gave dinosaurs their girth. The shrinking process started 210 MYA and proceeded apace for 50 million years. This appears a reversion back to the earliest dinosaurs, who were quite small.
Miniaturization afforded flight and the novel ecological opportunities that went with it. Besides the aerodynamic advantage of carrying less weight, smaller animals can flap their wings faster than large ones.
After growth slowed to keep size down early in their evolution, avian growth sped up again, to rates even faster than the extinct dinosaurs they descended from. This let a bird mature quickly. By the end of the Cretaceous, a bird the size of a sparrow could grow up in a week’s time.