"...2002 UltraPortable, 3600 mAh in 330 grams. In 2014 the extended battery [amazon.com] for the Sony Vaio Pro 11 is 4690 mAh in 290 grams, that's about a 75% increase in power/gram in 12 years."
Most of the difference is that older batteries were pretty much just a set of AA-sized batteries wrapped into a plastic shell. Modern computers often use pressed and formed LiPo batteries that allow for more "battery" in the same amount of space.
Others, like Apple, carry it a step further and completely eliminate the plastic shell used in removable batteries. I'll leave the math as an exercise for the reader, but you'd be surprised at just how much volume you gain by simply extending your battery size by 4mm in every dimension.