You may wonder what the man who's able to breathe every possible shade of meaning to "I am Groot" thinks about having to voice the character as a tiny sapling. And the answer is that Vin Diesel thinks there's a lot of differences between baby Groot on Guardians of the Galaxy 2 and full-grown Groot in the original, and hearing him explain them is simply delightful.
Speaking with Collider, Diesel gave this genuinely wonderful explanation:
Yeah, there is a difference. He couldn't be more naive as Baby Groot. I always think of the Groot that we saw on the first Guardians as a college-level Groot, so he's not fully grown yet, but he's a man. And once he sacrifices himself at the end of the movie, now he has to start all over, so to speak, but we're gonna see this goofy, adorable, baby Groot thing running around the screen and just learning as he goes.
It's interesting to note that the Groot we saw in the original wasn't a fully-actualised adult Groot, but the equivalent of a university student.
Watch Guardians again thinking about that.