"They like to watch the large elephants."
Translation:Ils aiment regarder les grands éléphants.
"Grand", meaning large or big in size, always goes before the noun.
Give this a read to help you understand the placement of certain adjectives before the noun rather than after: http://www.dummies.com/languages/french/how-to-place-of-french-adjectives-correctly/
Surely if la baleine est grosse so is an elephant? Not grande! Double standards?
I agree, it's inconsistent and "ils aiment regarder les gros éléphants" is also a correct translation.