According to a gunrag column; the lead-free priming compound Winchester (also Federal) uses has a very quick burn rate even as priming compound goes. Less mix is needed so the small primer size is adequate. Less area for the primer pocket pressure to act against reduces the risk of blown or backed out primers, so the small primer is more suitable here, too. I have read that they are crimped in but the ones I have fooled with were very lightly crimped, if at all.
It is mostly an aggravating contamination of the range brass pool. I haven't run into many (yet) and am just throwing them in a box to accumulate. When I get enough to bother with, I will load up a batch. Winchester says not to, without giving a reason, but handloaders are just natural tinkerers and I have tested some. A standard small pistol primer will give lower velocity than a standard large primer in a normal case. A magnum small pistol primer will make up some or all of the difference, or even gain velocity, depending on the powder used.