Simple and complicated.

A lighter bullet must be driven faster to give the same penetrating qualities of a heavier bullet.
All things being equal, with respect to caliber and velocity, a heavier bullet has a longer sectional density than a lighter one. The added weight and density help give it more punch.

I have found that with the proper load, a 38spl bullet has more thump than a 9mm bullet. It has more case capacity, and the ability to use bullets up to 200 weight.

As for small vs big, such as a 9mm vs 45 Acp, well, all they are doing is making a small bullet act like a big bullet by trying to get it to expand and drive up the velocities.

While small calibers are effective with the proper velocity and bullet construction, it is depending on variables such as speed and expansion to, well, act like a bigger caliber!

As for the use of reloads for SD; this has been going around for years. The assumption that reloads will get you into trouble legally was based on a specific set of incidents reported by Mas Ayoob years ago. Mas was not incorrect in what he said, but the application of the intent has been misunderstood by the masses.

In a SD situation, if the use of deadly force was justified, it makes little difference if you used a hammer, axe, Louiville Slugger or crow bar or knife to defend yourself. The same goes with handloaded ammo.
What sounds more ominous, a lswc bullet handload, or " Guard Dog, Zombie ammo, or Black Talon?

The problem cited in one incident Mas talked about was a suicide where the victim used light handloaded ammo made by her husband to allow her to shoot the gun for SD. Since the ammo didn't leave powder residue or burn make characteristic with crime lab testing, the distance of the gunshot became an issue.

Yes, anything can happen. But using handloads for SD is not in itself a determining factor in lawful use of deadly force.