Yeah, I dont think its quite as simple or straightforward as 25% less weight > 25% smaller batts = same runtime available. As weight increases the power required to maintain a certain desired speed increases at a faster/higher rate- for example:
my Dad's old Vauxhall Astra 1.2L could manage 100mph ( good times
), yet a Veyron with an engine many many times larger and about 8-10 times as much HP can only manage a speed 2.5 times faster....