Given the same volume, a tablet has more space for batteries than a laptop. A laptop has to use space for the hinge, keyboard, trackpad, peripheral ports, and display. Worse, you can't fit batteries behind a laptop display without ruining the balance. On the other hand, a tablet is practically a screen with a ton of batteries behind it.

Here's some real-world data: The 3rd gen iPad has a 42 watt-hour battery. The 11" Air has a 35 watt-hour battery. The Air is 25% larger in one dimension and over 50% heavier, but it still can't fit the same amount of batteries as the iPad.

And of course tablets typically have slower processors, less memory, and less flash than laptops. Their lower specs mean tablets draw less power.

With so much going against them, I'm surprised that some laptops manage to come close to tablets in battery life.