[gyardley]

I've never really understood why Groot stopped being able to say more than his name - he could talk just fine until 2008 or so:

http://13thdimension.com/wp-content/uploads/2014/07/acstarlo...

(On the other hand, the fully-talking character was a bit of a jerk, so perhaps it's better this way.)

[elechi]

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.