I've wondered if its due to smaller animals having a stronger flight reflex and that the twitching is basically that flight reflex automatically kicking in after the conscious death of the animal.
I've also seen hogs twitch a lot with airgun brain shots. The time one didn't twitch that I have seen, the bullet I was using out of a .25 airgun exploded under his brain pan. It didn't actually puncture the brain. However, the brain was blood shot as if the concussion right under it caused hemorrhaging. Very much like what happens with hydrostatic shock on firearm lung hits. That led be to believe that its the act of actually puncturing the brain that causes the twitching. Often with firearms I think we miss the brain itself but hit so close with so much energy it still kills.
I've also noticed that the more of the brain that gets fragged during the shot, the less the animal twitches. A clean puncture makes the animal go down still for a few seconds then the twitching starts. Total destruction of the brain such as caused by an exploding firearm bullet seems to result in minimal twitching. If the brain is the source of the electrical impulses that causes twitching, it would make sense that the less of the brain is intact, the less impulses it can send out upon death.
The only thing I've seen neurologists say for sure is that the animal is dead when the twitching is happening. I don't think even they know completely what is happening biologically when the twitching starts.