The simple explanation is that the heavier the bullet for a given caliber, the longer the bullet will be for that caliber.... longer is more stable and more wind resistant, all other things being equal- as long as the twist is sufficient to stabilize the length of the bullet.
Copper bullets of the exact profile will not have the exact same BC as a cup and core bullet of the same profile because they won't have the mass to maintain energy as long as the heavier bullet in flight or to fight wind as well- velocities and other conditions also being equal...
