OK, let’s try this another way: they could have sold a car with a say 70kW battery. Instead they are selling it with say 100kW battery.

Am smaller battery would weigh less than a bigger battery.

Because they de-rate the battery via software, you are carrying a bigger battery which weighs more than a smaller battery, while you only get the mileage of a smaller battery.

Hence, you have deadweight in your car.