Compare the powder space in a 9x19 versus a .38 Special. Don't confuse how something IS loaded with how something CAN be loaded. In a strong revolver, the .38 Special is always going to be able to beat the 9x19mm.
Absolute BS. The loads in the Speer No 11 manual for 9x19 are listed at not to exceed 35,700 cup max chamber pressure. The .380 is listed at 18,900 cup and .38 special is listed at 18,900 cup with +P loads running 22,400 cup. Powder capacity of the case has absolutely nothing to do with the performance of the cartridge in this case. When 9x19 is chambered in J frames, you'll never find it in an alloy frame gun. The cartridge would pound such a gun loose in short order where the gun will stand up to standard pressure .38, what it is designed for, with maybe a light diet of +P.
Typical energy levels for .38 special from a 2 inch gun are 260 ft lbs for +P, around 200 ft lbs for standard pressure. Out of my Kel Tec P11 subcompact, the 9mm +P I'm shooting is 410 ft lbs! That's TWICE the energy of the .38 standard load! You're pushing a 115 grain JHP to 1260 fps. That's almost .357 magnum snubbie territory or closer to it than it is to .38 special.
The 9x19 is the power king in this comparo hands down, no argument. And, it does this while fitting up to 12 in a magazine in the Kel Tec, accurate, and easy to shoot! That's why I like the subcompact nines. They beat .38 ballistics, .380 ballistics, Makarov ballistics, carry plenty of ammo, and are quick to reload. I ain't worth a toot with a speedloader, much as I like my little .38s. A subcompact nine is like carrying an 11 round .357 snub gun that kicks like a .38.
Now, I'm not saying here that I think any of these guns aren't enough for self defense! I carry a .380 on occasion when I need to for concealment as well as a .38 just because I like revolvers. But, don't be buffaloed by the volume of the .38's case. It was originally designed for black powder, after all. It is plenty adequate for self defense, but keep it in perspective.
A little addition here. The .357 magnum is listed at 46,000 cup max working pressure in the Speer #11 manual. Now, the .357 is but one tenth of an inch longer than the .38 case, thicker case to contain the pressure, though. Not much powder capacity difference here, but the .357 pushes up to 750 ft lbs out of a 6" gun. The best .38 +P loads are a little over 300 ft lbs. So, the .357 is packin', out of a 6 inch gun, now, almost 2 and one half times the energy of the .38. Does this illustrate how working pressure matters? It goes beyond that, though. The .357 acheives these outragous velocity/energies with a slower burning powder. It burns at peak pressure longer down the barrel. Such powder is less dense than slower burning powders, meaning, by volume and weight, it takes more of it. So, the .357 makes use of that case volume. The .38 at its low working pressures, is only efficient with faster powders, however. That works against the .38s large volume, can't use it because it's working pressure limit is not efficent with the slower powders. So, it has an EXCESS of space in the case. The 9mm case volume is much more efficient with the faster powders and with the working pressures can out perform the .38, but it will never be a .357 magnum because it doesn't have the case volume to utilize the longer burning, slower powders at the higher working pressures. But, the rub is the .357 needs longer barrels to gain the advantage of that slow powder or it just goes to waste causing muzzle flash. In shorter barrel guns, pocket size guns, it isn't near as efficient. You are well under 500 ft lbs with most loads in a 2" barrel. A carbine? LOOK OUT! But, 2" guns aren't the perfect use of the cartridge IMHO. I don't know if I made that understandable, but there's just more to it than the size of the case. Internal ballistics is a quite complicated subject. It involves lots of combustion chemistry and mathematics I don't like being exposed to. ROFL! It is not easy to model and I've never cared, just follow the manuals and be safe and happy and keep all my digits.