[masraum]

I believe .380, 9mm, .38, and .357 are technically all essentially the same "size" (diameter). The difference is the cartridge, power and weight. .380 and 9mm are both really close to the same physical size. I believe the main difference is the pressure generated behind the bullet. A quick google seems to indicate that it's roughly 21k psi vs 34k psi (obviously, there will be differences between loads, then there's the whole +p+ and all of that stuff).

9mm spoons are generally larger than 380 spoons despite the cartridges being almost the same size. I assume that a big part of that is that there's more metal required to control nearly 75% more pressure.