I'll probably get flamed for asking this but
Nonsense ...It's an excellent question.
It's one of those types of questions where my first answer would be," If you got into reloading, we wouldn't be having this discussion".
Handgun case volume isn't a critical factor.
It hasn't been since the black powder days.
Powder weight and burn rate are the critical factors.
Those steer the chamber pressure.
Higher chamber pressures usually result in higher velocities.
Higher velocities result in higher energy.
Double the weight of a bullet and you double it's energy.
Double the velocity of a bullet and you quadruple the energy.
Here's a simple energy calculator.
http://www.ballistics101.com/muzzle_energy_calc.php
I like this one because it leaves out things that just confuse the issue - like calibers.
Here's a link to Alliant Powder's online reloading guide.
You can browse through here and find all sorts of different bullet weights and velocities in all sorts of different calibers and plug them into the energy calculator.
http://www.alliantpowder.com/reloaders/index.aspx
Let's take a .357 mag, 125 grain Gold Dot Hollow Point.
17.5 grains of 2400 gives 1400 fps.
Energy = 544 ft pounds
10mm - 180 grain Gold Dot Hollow Point - (GDHP)
8 grains of Unique gives 1138 fps
Energy = 514 foot pounds
So, you have on round that's .357" in diameter and weighs 125 grains and a .40" diameter bullet that weighs about 30% more @ 180 grains and they have very similar energy figures.
Let's go one step beyond and throw in a .44 Rm Magnum just for grins.
The .44 Mag case is big enough to nearly swallow both the .357 and the 10 mm.
7 grains of Unique gives 899 fps using a 240 grain cast lead semi wad cutter.
Energy = 431 foot pounds
(Here's the thing. I've been reloading for roughly 40 years. I bet if we upped the charge of Unique to 8 grains, we'd be well within safe levels. I know this for a fact because some of my other manuals say it's safe)
(I also bet the velocity would jump up to right around 1000 fps)
Just for the sake of discussion, let's look at that
8 grains of Unique under a 240 grain bullet @ 1000 fps.
Energy = 533 foot pounds
So there you have three separate rounds, all having very different case capacities and all generating about the same energy levels.
The powder - 2400 for the .357 and Unique for the 10mm and .44 mag, are different. 2400 is a slow burning powder while Unique is pretty fast burning.
Unfortunately, they don't list the pressures.
W/any hope, you can start to see why energy figures, at handgun levels, have very little meaning.
A couple.three hundred feet per second one way or another can have a very profound effect on an energy figure, but, "real world" performance isn't affected in the least.
It (energy) does have a small place however if you want to do an apples to oranges comparison.