Ok, if you saw the extended edition of the two towers you would know that merry and pippen drank the entish draft and grew to a height of 4' 6" (like shaq for a hobbit) however at the very end of return of the king they are once again the same height as sam and frodo. I know that they might've opted to do this for those that didn't see the extended edition, but it still would've been better if they were taller.