And the pre-teen version of Groot seen in the sequel’s post-credits scene is apparently a few years after, meaning we may see something similar to that from when he shows up in the next Avengers film.
James Gunn also addresses the overall timeline of the Guardians of the Galaxy, which will be important regarding Groot’s appearance in Avengers: Infinity War.
Guardians 2 is set back in 2014, just a few months after the events of the first film; this explains why Groot is still a baby.
They only have four years to catch up to the timeline of the MCU, so Groot just may not have much time to grow into a more fully grown version of himself.