[sung in your best Sesame Street voice]
One of these things is not like the other.
One of these things doesn't belong.
Can you tell which one is not like the other?

I knew they existed, I had just never seen one, before, much less had one in my brass stock. Doesn't do good things to a large primer trying to stuff it in a small pocket. For those who are numbers oriented, small primer pockets measure a shade over 0.17" while large pockets are a shade over 0.20"
The thought, I'm guessing is that small primers are cheaper and that the difference in small and large primers isn't that great. Or at least isn't as great as say 100 years ago. And that may work very well for low pressure small case rounds like the .45ACP. And it would simplify my primer stocking if one size fit all, but I'd like to think that larger primers with larger flash holes are better for larger diameter ammo, like .45 and .44s
Just be careful out there. Inspect, inspect, inspect.