Groot's appearance in The Guardians of the Galaxy Holiday Special and the trailer for Guardians of the Galaxy Vol. 3 has left fans wondering why he looks buffed.Groot has had a significant physical change in all of his appearances in the MCU, starting out as an adult humanoid in Guardians of the Galaxy, where he sacrifices himself and is later shown as a toddler in Guardians Vol. 2 and I Am Groot only to grow into a teenager by the time Avengers: Infinity War and Endgame roll out.He is then finally a buff-looking humanoid in his late teens-early twenties in the Holiday Special and Guardians Vol. 3.What kind of creature is Groot?Character historyIn the comics, Groot is an alien tree-like humanoid or Flora colossus (colossi) that is native to the planet X. The language of the beings from this planet is rather impossible to understand owing to their larynxes being stiff, and thus their speech always comes off only with three words, "I Am Groot."While it is only three words, it is in fact different sentences that have a deep nuance and tone. Only a few people such as Rocket, the rest of the Guardians, and Maximus the Mad of the Inhuman Royal Family have been able to understand what the tree-like humanoid is saying.Groot and Rocket Raccoon in Marvel Comics (image via Marvel)Groot and his species are ruled by Arbor masters who grant each new generation photonic knowledge via photosynthesis, which is basically knowledge gained by previous generations. This makes him and his people extremely intelligent, as they possess previous knowledge in addition to the knowledge they gain themselves.The photonic knowledge granted to them also reflects in their physical appearance.Is Baby Groot a reborn version or the offspring of the original?Left: Groot in Guardians of the Galaxy, Right: Baby Groot in Guardians of the Galaxy Vol. 2 (images via Marvel Studios)Is Baby Groot a reborn Groot from the 2014 Guardians of the Galaxy film or his offspring? The answer is his offspring, as confirmed by director James Gunn in a now-deleted tweet, in which he wrote:"First Groot is dead. Baby Groot is his son."How did Baby Groot become buff?Groot in The Guardians of the Galaxy Holiday Special (image via Marvel Studios)James Gunn explained the decision to make the character more buff on Twitter. He mentioned it was done to make him slightly different from his father in the 2014 film as he is, in essence, of the same build. Gunn also stated that "Swoll Groot" has a more outgoing personality and a stockier physiology.James Gunn@JamesGunnWell, YA Groot, or Swoll Groot, as we call him, is getting bigger, outward as well as upward. As you can see by now he has a different, stockier physiology (and a more outgoing personality) than his father did. #GuardiansoftheGalaxyHolidaySpecial twitter.com/jetblacjetblac…nope@jetblacjetblac@JamesGunn loved the special, especially nebula's dancing was left wondering if groot's head is so big because he's still growing into it or because that's just the way this Groot is and not at all like the slim OG Groot!4261193@JamesGunn loved the special, especially nebula's dancing 😆 was left wondering if groot's head is so big because he's still growing into it or because that's just the way this Groot is and not at all like the slim OG Groot!Well, YA Groot, or Swoll Groot, as we call him, is getting bigger, outward as well as upward. As you can see by now he has a different, stockier physiology (and a more outgoing personality) than his father did. #GuardiansoftheGalaxyHolidaySpecial twitter.com/jetblacjetblac…It is unknown how or exactly why the young adult Groot got a bulkier physique. It has however been theorized that due to him spending time with Star-Lord and Mantis, his two teammates with celestial DNA, known for their radiation, he has taken in both those DNAs which ultimately ended up informing his current look.Thus, ultimately, the character's appearance is a combination of his natural state of progression and James Gunn's creative decision.