Mainly improvements in materials and processing I believe, they do state something to that effect. It's mostly for things like metal film resistors, not so much Carbon types. Whereas the 'mini' resistors used to be 1/16th or 1/8w, the are commonly now 1/4W and what used to be 1/4w size is mostly 0.5W now, some we get are 0.6 and I've even seen 0.7W is the same size.
The tend to be able to tolerate higher temperatures, presumably more flexible with better terminations and other improvements.
Take this one for example, they now rate it at 0.6W, but when I started in electronics, this would have been a 0.25W size. The 'Range Overview' says "This series of resistors is now rated at 0.6W (commercial spec) and 0.25W (CECC military spec).
Axial - 0.5 - 0.75W & Zero Ohm Links. High stability metal film resistors. Extremely low current-noise level with low temperature coefficient and close tolerance." http://australia.rs-online.com/web/p/through-hole-fixed-resistors/0148506/?origin=PSF_433011|fp
It's been very common over the past decade or two to see ratings for sizes like that doubled or even more, but it's more about the quality of the part.
Likewise Electrolytic Caps have shrunk dramatically over the years for a given size and capacity.