Two different things here- related, but different. Energy is mass(in grains) x speed (feet per second) squared, divided by 450411. For the math challenged, here is a calculator:

Muzzle Energy Calculator
The answer is in ft lbs- not lbs per square foot. One foot lb is equal to one lb falling one foot.

HOWEVER- then we come to terminal ballistics- how well the projectile transfers energy to the target. IN GENERAL- the bigger cross section transfer energy more efficiently. Which explains expanding bullets, such as hollowpoints. But then you have rounds that de-stabilize on impact- such as .223, the Brit 38-200, etc. And pre-fragmented rounds, such as the Glaser Safety Slug, etc.

So a 25-06 and a .25 ACP have the same diameter bullet, but the .25-06 is heavier, and much faster, and has a lot more energy. A .308 and 30-06 may use same weight/ diameter bullet, but the 06 is faster- has more energy. A 158 grain .38 Special is 21% heavier than a 125 gr .357 mag- but the mag is moving much quicker- and has more energy.

This discussion is a long one, well suited for a least a 6 pack by the fire. Who's got the pretzels, anyway?