The capacity of the battery is measured in mAh (Amper hour / 1000).
The improvement allow the production of batteries with more and more density of energy.
For instance the K700 was designed and produced about 3 years ago now.
Today is possible to produce the same battery with higher mAh.
Even if SonyEricsson doesn't invest in old project it's possible to buy 3rd part replacement with better performance.