When you give up weight you gain velocity, and vice versa, for the same powder load.
I just ran the ballistics of two different .40 SW JHP cartridges. The energy depends on the distance; the crossover is at about 10 or 12 yards out. But the velocity crossover is quite a bit farther out. The energy of the bullet involves both the mass and the velocity, but the velocity is more important. On the other hand, air resistance slows down the lighter bullet faster. It isn't as easy as just deciding on the bigger bullet.