Is the only difference in a large pistol primer and a large rifle primer the amount of igniter in the primer? Thanks
No. When it comes to the large primer sizes, the difference between rifle and pistol is also in the primer height. The large pistol primer is about .008" shorter than a large rifle primer. So, you can not use a large pistol primer in a round where a large rifle primer is needed, and vice versa. If you do this, one of two things can cause a problem. If you seat a large pistol primer flush or just below flush in a rifle case, the anvil will not be bottoming out in the primer pocket correctly and can cause the primer not to ignite. The other problem is if you seat the large pistol primer in a rifle case and you do seat the primer deep enough where the anvil flares out and makes proper contact with the primer pocket, your primer is now seated too deep for the firing pin to reach the primer correctly.
In either situation, it can cause a problem with proper primer ignition. And, yes, there is a difference in the primer compound amount, meaning how hot the primer is. This is for the large size primers.
The small rifle/pistol primers are the same height, so they are externally the same. The main difference would be the primer cup thickness and the amount of primer compound in each primer.