A hybrid has all the same ICE components with the addition of the generator and battery pack so it's not surprising that it would be heavier than the pure ICE counterpart. These kits will be replacing old iron block engines (along with the other aforementioned drivetrain components) with a small aluminum engine and generator. Even if they are a bit heavier, these are old trucks with poor aerodynamics where a few hundred pounds isn't going to make much difference in the grand scheme of things.