Sorry, I should have added this to my statement
the higher the powder charge required...to maintain the same velocity...
You put a larger something, anything in a finite space then one of the other items must be reduced...more shot=less powder or smaller wad/cup; heavier (which means longer mostly) bullet then less powder; more powder then less (which means shorter mostly) bullets...
Weight of projectiles can also be altered by type of material used to produce them...
Adobewalls produced a very succinct comparison but that translates to paper performance...The type, material, shape etc. of the projectile can and will change the equation, dramatically...A 140 gr FMJ bullet will penetrate a lot further into the body cavity then a 140 gr JHP but that will go deeper then a 140 gr frangible HP bullet...
Lethality is determined by velocity, mass, penetration and expansion...
The mushrooming effect of the bullet is what causes the massive wound channel, greater damage to the organs, tissue and veins and that causes the massive shock and bleeding...Less blood flowing means less oxygen getting to the brain, less brain activity, slower/weaker they become until the brain shuts down completely and death occurs...
Bullet designers nowadays have come up with expansion based upon velocity upon striking the target which translate to depth of penetration...While brain shooting an elephant requires a solid bullet to get through the huge bone and then past the mass of muscle to lodge deep enough to shut the brain off, expansion isn't that important but mass, shoulder (diameter) and velocity is...Whereas a shot to the chest of a deer you want to dissipate all its energy within six inches or so, expand to huge width as that will kill but the more solid projectile will go clean through the animal and cause very little damage--in many cases none--and which is why FMJ bullets are sanctioned for military use by the Hague Convention, translates to survivability...
Take a bullet and a prairie dog; shoot the one at the other at 4,000 fps and the PD is a red mist floating in the air, 3,000 fps it does a triple back flip landing 5" away, 2,000 fps a double axle in place, 1,000 fps it flops over dead and at 500 fps it just may crawl away and die painfully--* presented for informational purposes only and not specific to any calibre, cartridge or bullet type or make!