Ain't it because the original old Groot died in the first Guardians then he planted a seed which was his son and then his son was baby Groot in Infinity War and Guardians 2 he was a teenager Guardians 3 he's like a young adult now.

I don't know for a fact, but um, my guesstimation, my theory is that maybe, you know, uh, being exposed to Thor's power, uh, had an effect on Groot and, and, and made him buff Groot. You follow me? I mean, I could be wrong. I mean, I don't know what the canon is. I have to look it up to see if anybody has, uh, has verified, you know, what the reason is we got buff Groot. But, uh, I thought it was cool, man. I thought it was cool. And I thought it was cool that we finally got to understand what the hell he means when he says, I