Length is a dangerous element to tackle. It takes time to design levels, and the more there is, the more time it takes. If they just carelessly try to extend gameplay, then it likely receives "repetitive gameplay" criticism, or poorer quality in general. Remember too that, the longer a developer takes with their new fancy engine, the more their fancy engine becomes outdated even before it hits the market, as what started to happen with Half-Life 2's Source engine.
Old-school games like Super Mario Bros. 3 can be beaten as quickly as under a half-hour for a non-speed-running person, and less than 8 hours if they don't skip anything (this is assuming there's a fair number of deaths too). Portal -- an awesome game -- can be beaten in less than 4 hours. I would replay those games over and over again far more than even most lengthier-than-normal games.
The Plothole: a home for amateur, inclusive, collaborative stories
http://forums.theplothole.net