I couldn't find this finance meaning on OED or Merriam-Webster, probably for it's too newfangled? Recently SoftBank has been blazoned as the NASDAQ Whale.
I'm not convinced by the explanation in the first quote below, for while whales can splash, their splashes aren't huge in the context of the hugeness of oceans. A whale's splash affects the ocean far less than a financial whale can affect a stock market.
Understanding Whales, Bulls & Bears | by Stably | Stable Trade | Medium
Whales
A whale is any individual or company who has enough money and power to directly influence the price of a cryptocurrency or stock, usually in a negative way. Think of a whale and their large mass. They can make huge splashes and the same concept can be applied to crypto/financial markets.
Investing 101: What are whales and how do you profit from them? — Steemit
whales has become a term associated with market manipulators. Market manipulators will come into a market, buy a majority of the available positions or limit orders and drive up the price. Once the price has reached their target price, they will attempt to liquidate their position, albeit a large position which may take some time. Generally, this will cause the price to crash back down to it's inherently acceptable level.