Well, obviously nobody expected The Empire Strikes Back to make as much money as Star Wars. Nothing had ever made as much money as Star Wars except for Star Wars, and everyone in 1980 knew that sequels always made less money than the original film.
In 1972, The Godfather was the highest-grossing movie of all time with a $134 million domestic gross — but Part II, released in 1974, only made $47 million.
Then in 1975, Jaws became the highest-grossing movie of all time, taking in $260 million at the box office — and followed it in 1978 with Jaws 2, which made $78 million.
The Exorcist II made half of the first film’s gross; ditto for Smokey and the Bandit II, and even more so for Damien: The Omen II. The Airport sequels dropped $20 million with every release. Herbie Goes Bananas was the fourth film in the Love Bug series, and I think they actually ended up owing the audience money.
The Superman films followed the same pattern: the first movie in 1978 made $134 million, Superman II in 1981 made $108 million, and in 1983, Superman III made an embarrassing $60 million, which didn’t even crack the top 10.
But that same year, the Star Wars series did something surprising: the third film actually made more money than the second one did. Star Wars got $307 million in 1977, The Empire Strikes Back made $203 million in 1980, and then in 1983, Return of the Jedi made $253 million. So obviously George Lucas did something right with The Empire Strikes Back and Return of the Jedi, because his series made more money as it went along.
Still, money isn’t everything. Oh, wait, of course it fucking is.