Sqrl, it isn't always about the amount of content, though.
A classic example is Super Mario 64 to Donkey Kong 64. DK64 had much more content. The levels were WAY bigger, there were tons of characters with a huge variety of moves, and there was way more to collect, with most of this collection involved exploring new areas.
But which one do people probably get more play time out of? And which one feels more tedious?
In SM64, you want to keep replaying old levels. Most of the stars can be gotten by slow, careful play, but with the feeling of perfect control over Mario, and the ability to string his jumps together in different ways, it's fun to keep playing, finding short cuts and different routes, training your reflexes, running through platforming sections full-tilt, etc. I've spent hours in that game doing things as silly as finding places I can lose all my health while running off a cliff, so that I get kicked back to the castle hub and die there, and get kicked back to the bridge outside. Stupid, ridiculous fun.
In DK64, there isn't that feeling of perfect control, and there aren't all these different paths through the environment you can find by stringing moves together in different ways. The vast variety of moves eventually feel like 40 ways to hit a switch and open a door, and knowing that every challenge inevitably leads to another banana makes no challenge feel particularly worth your time. The game is paced and designed well enough that there's motivation to beat King K. Rool, but I've never felt an urge to collect every banana afterwards.
Why do you think the 3D Mario platformers haven't each upped the ante for how many stars there are to collect? It isn't ABOUT more content, it's about making the player WANT to play.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.