Unless the vendor changes to some new kind of technology there's simply no way they can get more capacity into a same-size battery. With lithium-ion and lithium-poly standard technology it's a no-brainer what capacity you get given the volume of the battery.

Every (_real_) high-capacity battery I've ever heard of are always physically larger than the lower-capacity original. Take for example the high-capacity 3party batteries for the Treos, they need a replacement battery cover because the battery is thicker. The higher-capacity battery I put in my Palm T3 is also larger than the original, and the only reason it (barely) fits is that it's a lithium-polymer which can be formed like clay.

In short: If you see a replacement battery which claims higher capacity but still has the exact form factor as the original - don't bother. It's simply not true, unless they explain what kind of new type of battery technology they're using.