Self-answer to my own question :
I 've just read on the 2004-prius yahoo group that someone has been able to discuss with Dave Hermance, Toyota chief engineer, about the 2004 Prius at Michelin Bibendum Challenge. He said that although the total amount of energy was lower in the 2004 battery, the available energy is about the same because Toyota engineers learned from the current generation that they could use a greater charge range (the cells used are a newer generation too). Therefore, with a smaller, lighter, less expensive battery but which stores (in practice) the same energy amount and delivers the same power, the new Prius achieves higher performance and efficiency levels due to many improvements (body shape, engine and motor efficiencies, electronic control...).
Cool... 8)