I’ve noticed this too. Popular movies from the 80’s and 90’s were standardly around an hour and a half (no statistics, just my observation) while today’s big blockbusters are rarely under 2 hours, usually 2 and a half (or more). And it often feels like movie makers are just making the films longer for the sake of meeting “today’s standards”- it rarely seems necessary for the story itself.
Inception and Avatar come to mind immediately.
Rain Man, Die Hard, Jurassic Park, Terminator 2, Titanic, Independence Day, The Fugitive and lots of others are all over two hours. They all made huge money and they were all released in the 80s and 90s.
Run times on the Marx Brothers’ best movies:
Monkey Business - 77 min.
Horsefeathers - 68 min.
Duck Soup - 68 min.
A Night at the Opera - 96 min.
Or Bogart:
Maltese Falcon - 100 min.
Casablanca - 102 min.
To Have and Have Not - 100 min.
Key Largo - 100 min.
Beat the Devil - 89 min.
A lot of not-so-long movies.
The answer to the question is double features.
Heh. This is why I’ve stopped buying a large soda when I go to a movie. I got tired of my bladder being ready to explode right as the climax of the movie was approaching.
For the OP, I figure old movies were shorter simply because the actors all talked so fast. It got the scenes over with more quickly 
Because the acting’s so bad?
Double features was part of it, but so was the studio system. Top directors, like the ones making the top rated movies listed, did A level roadshow movies. Most directors got assigned films, and the studio wanted to churn out as many as possible, which means they were short. And often parts of series and done with contract players.
When I was a kid not only were there double features, but suburban theaters charged admission, and did not clear the theater out between shows. You could see both movies for one price, or for single movies see it multiple times if you wished. You could also come in in the middle if you came late.
In addition to features, there were also usually newsreels, shorts (sometimes in serial form), and maybe a cartoon. The feature wasn’t the entire moviegoing experience.
Theaters also had a single showing each evening. There would be the shorts, cartoons, newsreel, previews of coming attractions, and a film (or two). You could come whenever you wanted. It was the equivalent of spending an evening in front of the TV.
In the 50s, they started having two showings, and later there were continuous showings.