The main reason whales are able to grow so big is because they live in the ocean. It is, in fact, no coincidence that the largest animals living on earth live in the ocean.
Scientists believe that the weightlessness of the ocean makes large body size possible.
Otherwise, the skeleton could never support such a weight on land. Another factor making whales so large is their diet. Basically, the lower a predator feeds from the food chain, the more energy it will gain from the food. Large whales, being top on the food chain, feed very very low, on things that even fish and other lower animals feed on. What this means is that all the energy krill has packed in its little body get transferred right over to the whale.