Re-reading this thread,, I also failed to mention another factor in relation to accuracy. And since we are discussing 38/357 types of ammo, I'll use that as a base.
Cylinder alignment.
If the chambers of a cylinder are not in good alignment,, accuracy can suffer. A shorter cased ammo, may or may not allow a bullet engage the rifling before it fully exits the case body. If the bullet is shorter,, and the case is shorter,, then the bullet enters the chamber area, then enters the throat then the rifling. Often,, the SLIGHT cylinder side-to-side play can allow the alignment issue to be easily overcome. But a longer case, along with a longer bullet can often allow a much easier "transition" of the bullet from the case to the rifling.
Hvymax's pictures above show a long bullet, near the chamber mouth, and will be very close to the throat & rifling. As such,, it'll align quickly.
And yes,, as mentioned in an above thread,, load density in a case can & often does have an effect on velocity and accuracy.