As far as I can see, you can slide the bullet diameter and weight up and down the scale all you want with roughly the same powder charge and get the same outcome.
The overall force behind a round's penetration ability comes down to its forward inertia, and that's a simple case of Mass and velocity. To retain inertia you need mass, but the more mass you add with the same force you lose velocity. Thus you get light 9mm rounds with less mass and more velocity performing not much different from .45acp rounds which will have less velocity but more mass.
If you go too far to the light extreme you can get your projectile faster but don't have enough mass to retain the inertia and it quickly loses inertia and with it penetrating capability. Conversely if you get too heavy without increasing the charge propelling the projectile you don't get the projectile moving fast enough to build up any forward inertia. 9mm, .40S&W, and .45acp (along with most other standard power rounds in the same category) all seem to slide up and down the scale in the same safe zone of moderate effectiveness.
This is why I stand by magnum rounds. To get any real effectiveness you need both mass and velocity. In other words a heavy projectile moving at high speed and with that carrying a ton more inertia. Thus a hot loaded .357 magnum or 10mm round with a heavy projectile is going to outperform anything with less charge behind it regardless of caliber. It's simple force=mass x acceleration
If you can't carry or handle a more potent round, then just carry whatever in the range you shoot most confidently. I could maybe argue that wider diameter round will produce a slightly larger wound, while a smaller round will give you more shots, but that's an argument that's been made plenty of times already.
"Use more gun." -The Engineer- (Team Fortress 2)
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