While I agree that the width makes a difference, there is another reason they are able to cram a battery that size into such a thin form factor. The Razr Maxx, like the Droid Razr & the iphone, are sealed units meaning there is no cover to remove to allow for swapping the battery. By doing this they have much more space available. Basically they can solder the battery right to the circuit board. No battery tray is necessary between the board and the battery. Also with no tray the battery can reach all the way to the casing of the phone.
Another thing to look at is the design of the Razr and the Rezound. The Razr has right angles at the edges, where the Rezound has the edges sloped in about 1/4". Which means the battery has to be 1/2" narrower total than if it were designed like the Razr. Just look at the extended battery for the Rezound. It's not very wide or tall, kinda like a little puck. If you were to make a copy of the Rezound's extended battery out of clay, and then squash it down to the same thickness as the Razr's battery, and then set it on top of the Razr's battery, I'd bet the Razr's would be larger.
Now while the Razr design (both visually and not having to have a battery tray for removal) allows for a larger battery, it felt very awkward in the hand (it's not just me, there have been quite a few in these forums who feel likewise). It basically rested against the thumb and the center of the fingers, forcing most to stretch to hold it. I only had to hold it for a minute and I instantly hated it, so I cant imagine being on a long call with it. The extra thickness of the Razr Maxx may make for a better feel in the hand, but to some of us it's just to damn wide. The narrower width of the Rezound along with the sloped back makes for a much better feel in the hand IMHO.
The last thing to consider is that the 24 hour lifetime is just a claim. I know they are having an event to showcase that it can do it, but I would bet that there will be excellent signal in the area where they are testing it. Now if they would mix it up a bit and try to reproduce a regular day with fluctuating signal from -65dBm to -105dBm that would carry more weight. It's no different when other manufacturer battery claims are made. They have the phones in a lab with a good strong signal, rather than just taking it out and talking on it while driving around or even doing the testing with a middle of the road signal level, like say -85dBm. The only way to know for sure what you're really gonna get is to try it yourself or look for reviews like Phandroid or Engadget do where they do a battery rundown test in their office which is subject to environmental factors that are not present in a lab setting.
