That's not my experience. Many times magnum primers have resulted in considerably better accuracy when using powder charges under 75 grains, and sometimes with far smaller charges. Often that occurs with spherical powders, but sometimes not.
One good example occurred the first time I used Ramshot Big Game over 15 years ago. The rifle was a .220 Swift and the powder charge around 40 grains. Velocity was great but 5-shot groups at 100 yards were well over an inch. Switching to a magnum primer cut group size in half.
In magnum-sized cases a good example is my custom .338 Winchester Magnum. With 200-grain Ballistic Silvertips, 67 grains of Reloder 15 and magnum primers, average accuracy for 3-shot groups is around .6 inch. With standard primers accuracy is very mediocre.
Could provide a bunch of other examples.
