From there the film just clicked, everything fell into place and Baby Groot just popped.
In the earliest drafts of the script, Groot was fully grown again, but Gunn said that something was still missing and he had trouble finding a place for him.
Though it’s easy to write of Baby Groot’s inclusion in the movie as a means to sell more lunch boxes, director James Gunn revealed that originally, Baby Groot wasn’t even going to be in the movie.
Then he decided to set the sequel a few months after the original, which meant shrinking Groot down.