Prof Young,
There are different approaches to making a primer magnum. Some make it burn hotter and faster by just increasing the amount of priming mix. Others change the mix to add fuel components so it makes more gas for pressurizing a large case, but actually has lower brissance. The WLP primers may be of the latter sort, staying gentle enough for target loads, but making enough gas and flame for magnum loads. As near as I could tell when I used them about 20 years ago in the .45 ACP for target loads, Winchester WLP's presented no significant performance change over my more usual Federal 150's. I notice they've not made a standard/magnum combo primer like that in any other size, so it seems like it's something that only works out for them in the Large Pistol format with their priming mix.
Read
this primer on primers.