Record returns for “Joker” have the movie’s backers laughing their way to the bank, but the studio that made the film, Warner Bros., could have reaped even bigger profits if it hadn’t unloaded a substantial share of the film to outside investors, according to people familiar with terms of the film’s financing.

Now the top-grossing R-rated film of all time, “Joker” reached a total of $849.1 million world-wide over the weekend, according to Warner Bros., a unit of AT&T Inc. The previous record of $782.6 million was held by “Deadpool,”...