Many kydex mag pouches have tension adjustment screws, like Blade-Tech and Kytac. It's pretty easy to make them fit a wide range of magazines. I use my Kytac pouches for my P30 and Glock mags.
I would be very careful using a leather pouch, without adjustment screws, for magazines slightly larger than what it was made for, you may end up stretching it enough so it's too big for the original mag.
Ok, so I guess my next newb question would be why would one prefer one type over the other? Is it more an aesthetic choice? Function? Comfort?
I don't have a material preference, I have a size, ride and adjustability preference.
Ride: I like mag pouches that aren't too bulky and keep the mag close to my body (for concealed carry).
Size: I like them to be about 2" deep. I want to get a really good grip on the mag before I start removing it from the pouch, having less of the mag covered up helps for this. I also like shallow pouches so I can get my index finger on the bullet tip more easily - which helps me with indexing it for mag changes.
Adjustability: It helps me keep fewer pouches on hand (less $$ spent) for when/if I change guns.
Personal preference and application drives the decision making. For range and competition, it's kydex for me. For concealed carry, I prefer leather for both holster and pouches due primarily to comfort. For duty, the decision is already made for me, it has to be leather with fold over enclosures.
As for your question, why the difference between kydex and leather. It's already been summarized, "some" kydex manufacturers have built in retention screws where other's do not. As for leather, it has more give; however, permanent stretching can occur when using a magazine that is too large.
Frank B
*************
"You are not paid for what you do, you are paid for what you may have to do, and when that time comes, you will be highly underpaid."