You'd expect Avengers: Endgame to be longer, since it's the culmination of this story. As Joe Russo said, it's a big movie with a lot to cover. Someone has to save Tony Stark, not to mention try to save the half of the planet that was snapped by Thanos. Hawkeye has to rejoin the story after his Infinity War absence. Scott Lang's Ant-Man has to tie himself to the Quantum Realm plot. And of course there will be a lot more footage than Marvel doesn't show us in any upcoming trailers.