> Is this a 'when batteries become magical' thing?
Basically, yes.
To get more power you need a bigger battery, but if you have a bigger battery you are carrying more weight and thus need more power, so you quickly reach a point of diminishing returns.
Batteries are improving (and stronger/lighter materials to build with are helping as well), but nowhere near the sort of Moore-ish improvement we are used to seeing in other places.
Basically, yes.
To get more power you need a bigger battery, but if you have a bigger battery you are carrying more weight and thus need more power, so you quickly reach a point of diminishing returns.
Batteries are improving (and stronger/lighter materials to build with are helping as well), but nowhere near the sort of Moore-ish improvement we are used to seeing in other places.