Right.
If you throw a projectile on say the moon, a light one would go the same distance as a heavy one if both launched at the same velocity, but the light one would have less energy. On Earth, assuming the same size and shape, air resistance losses would be fixed so the lighter one would lose more of its energy PROPORTIONALLY and hence slow down more.
So lighter projectiles don't go as far launched at the same velocity. If you are firing from a gun then it's slightly different since you're launching at the same ENERGY (sort of but probably not in a real life gun situation) not the same velocity. The same energy into a smaller projectile gives greater velocity and hence further travel provided it's not too light to lose too much to air resistance.
If a projectile split in mid air, you have to ask how is it splitting? If there's some kind of charge in the middle of it firing half backwards and half forwards, then the forward part would gain the momentum of the backwards part so would go faster. But the charge would add energy into the system.
As thepurist suggests, if the projectile simply broke up they'd both describe the same trajectory so nothing would change - provided one part was not so small to experience significant losses to air resistance. If it was a small bullet sized projectile and it broke into equal parts you'd have two pretty small projectiles and the above mentioend effect of air resistance on small projectiles would make them BOTH travel less distance.
