Sandisk's next mp3 player?

I don’t quite understand Sandisk’s mp3 player strategy when it comes to battery life. Its players are generally well liked, except for the battery life. Why doesn’t Sandisk use a larger battery in its players? Doubling the battery life won’t make the players that much larger. I think for a player like the Clip Zip, using a battery with double the capacity might only make the player around 20-25% thicker. Having a Fuze with 40 hours of actual  battery life(stated battery life around 50 hours) would make it so much more useful than having just 20 hours.

Having two card slots would also greatly increase its flexibility.