The youngest ones almost like the Littlest Pet Shop characters - This header picture illustrates it well. 

My answer is: to make them look kind of like Toys :) . Just look at little childrens' ( around Preschool age ) drawings: They draw characters Exactly Like That!... - They don't have the 'sense of proportion' developed yet - so they draw even Humans with Such Big Heads! 😉

So I guess it's made to meet Their Perspective - & to make cute Trolls faces with nice eyes & wide smiles more visible :) ... 

( I saw some Trolls fan drawings with more 'human proportions' - & then some of their cuteness was Lost! - they didn't look That Adorable!  ^_~ )