We all know that an ideal bullet is one that provides massive expansion and still has enough weight leftover to exit an animal. Better said than done!
Granted, most bullets are on one end of the spectrum or the other, but manufactures are getting much better while moving closer to middle ground with adequate expansion / penetration in most cases.
Hunters should always look for penetration first and expansion second because an exit hole = a good blood trail and more than likely a recovered animal.
Moreover, just because a bullet expands and penetrates well on light-skinned deer, doesn't mean it's automatically adequate for elk. I.E. Remington Core Lokt, Winchester Power Point, Sierra Game King. Conversely, just because a round does well on elk, doesn't make it a deer bullet either.
Example of all-around performing projectiles:
Bullets from Nosler (Solid Base / AccuBond / Partition), Speer (Hot Core / Grand Slam), Hornady (Interlock / InterBond), Remington (Ultra Core Lokt), usually provide the best of both worlds (good expansion, plus exit wound); performing well on deer or elk as long as the right grain is used.
While a bullet may have the ability to expand and take a deer, remember that in order for an exit wound, light for caliber rounds MAY need to be avoided.