My 2cents. I know diddly about cellphone batteries but what I've noticed since I received today. The seido is bigger. But just marginally. I noticed the stock battery would just drop into the battery compartment, but the seido needs to placed in just right as the fit is very snug. You don't need to force it though. Second, its noticeably heavier. Not blackberry weight but it does make the inc feel noticeably heavier. So making a guess, and this is only a guess. It's possible that they simply maximized the amount of space and put in more material. I know there's doubts about how can you make a better battery that's the same size and get more juice. Now I know this is apples and oranges but the optima battery that replaced the stocker in my trick is at least 20 percent smaller yet packs a help of a lot more juice. Secondly people are saying HTC would never put in an inferior battery in their phones. True, but let's talk margins. Maybe that little bit of extra capacity would have have cost them .03 cents per unit. Verizon obviously wanted to hit a certain pricepoint and HTC as well so make sacrifices where they can. Rather then skimp on the hardware, sacrifice the run time and offer an optional extended battery. It also makes a case for pushing the extended for battery for heavy power users. Cha-ching.
So far an hour of heavy internet use, no WiFi , no bluetooth, no gps, animated wallpaper and that fancy HTC weather widget on. 50 percent left.