It's a size thing. When he's big, his vocal cords are too stiff to really sound like anything other than 'I am Groot'. But, to be fair, the earlier version is

It's a size thing. When he's big, his vocal cords are too stiff to really sound like anything other than 'I am Groot'. But, to be fair, the earlier version is based on Groot being a monster, a villain. They re-imaged a lot of the Guardians of the Galaxy to have a more proper spaced based origin.