Jargon taken from the casino industry, a "whale" refers to a high-spender.

Historical perspective: Facebook profited from “in-game” purchases by children who used their parents’ credit cards to spend thousands of dollars, said Nathan Halverson in RevealNews.org. A federal judge in a class-action suit ruled in January 2019, that documents detailing the purchases should be made public. Complaints about kids racking up charges on cards that Facebook kept on file were so frequent that “Facebook employees began voicing their concerns that people were being charged without their knowledge.” In internal chats, some Facebook employees called high-spending kids “whales,” lingo taken from the casino industry. In one discussion, Facebook employees refused to refund $6,545 to a user who “looks underage.” Research by the social media firm found that the average age for players of Facebook’s version of Angry Birds—a game that allows in-app purchases—was 5 years old. 
See also : child tech addiction  
NetLingo Classification: Online Jargon