Chris Pratt's salary is basically part of the marketing budget. They probably had a list of A-List actors, maybe some lesser known ones too, did some calculations or put it into an all knowing algorithm, and it spit out his name where paying him X amount will guarantee Y amount for best investment. Just how movies are, especially animated ones.
I'm just very happy we have some amazing main cast choices in Charlie Day and Jack Black, plus Keegan-Michael sounds great. Even Seth Rogen might be a surprise hit, like I can't imagine anyone would have guessed him.