That's the big thing. I think the only reason why we think the older games are longer is a combination of no saving, in addition to being poorer game players.
I mean, if you think about it, all of the Mario games for the NES were made to be beaten in a single sitting.
In fact, most NES games were made to be beaten in a single sitting, unless it was a bigger game that had a battery for saving the game. I don't consider games that can be beaten in a single sitting to be "long" games.