When I got into R/C racing the third time around, LiPo batteries had just started to make their appearance in the hobby. A 2S battery pack for a surface vehicle-- that is, one of the dimensions to fit in a standard NiCd battery tray-- was 2200mah. Now, the same size battery is 6500mah. Or just look at 18650 cells, all of an absolutely identical form factor, but available in capacity anywhere from 2000mah to 3600mah.

They get better at the chemistry, so it's absolutely possible to fit a 1000mah in the spot for an 800mah from nearly a decade ago.