"Blue Whales are the largest animals in the world."
Ther above is from Kids' English book and I don't understand why it doesn't say
"Blue Whales are the biggest animals in the world."
It could, but it probably wouldn't. "Largest" sounds better in these type of cases.
I was reviewing someone's technical paper the other day and he had written about "big data sets." I corrected it to "large" because that is the word that is normally used. It just sounds better. "Big" seems more childish, at least to me.