A "whale" in cryptocurrency slang simply means the person on steemit is "a bigger fish than most of the other fish". Sort of like the old saying "a big fish in a small pond".
The term "whale" originates from gambling. According to realonlinegambling.com, competitive poker players would use marine-inspired categories to describe a person's playing skill and how much they had to wager. Regular small-wager players were "fish" and the expert players who preyed on the regulars were "sharks". Following this pattern "whales" are the largest creature.