Sadly the reason the battery can be so big in the Razr Maxx while remaining thin is because of their non-removable back plate design. If you check out one of the videos that shows it being disassembled you'll see the battery is nearly the size of the entire phone's footprint and I assume this is achieved by removing the traditional battery 'socket' that we see on a phone like the Nexus.
We may be looking at more manufacturers moving to the non-removable back plates to achieve these larger stock batteries if the Maxx becomes popular. I'm not sure if Motorola holds the patent on this thin battery tech, it looks very odd if you check out one of the videos showing it. It's almost as if the battery is flexible, but that may just be because it's so thin.
Razr's battery vs Nexus battery:
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