I think its also important to balance the need for penetration versus energy transferred to/retained in the target.

i.e A .177 at 15 yards passing straight through, for example, a squirrel thus unless a vital organ it hit the actual knockdown/damage potential is wasted as the pellet continues its flight. On the flipside and other end of the of the scale a .25 at the same range/target can also rely on the pellet remaining in the target thus the full energy is transferred into said target as well as the physical damage caused by a larger projectile.