Actually I took a peel and stick floor tile and cut two strips to the same size as the inside side of the mag carrier. peeled off the backing and stuck em in..perfect fit and retention..I just bought one today.
If it came in the Glock factory clamshell package, check the back of the package. It will say either LG or SM. You may have the large which is for the double stack .45acp mags. I tried both sizes while on the store and the small fits G19 mags perfectly with just enough retention.
I got mine from the Glock site store and there was only one size listed...they are for a 26 mag so I may have gotten the larger one and not known it as it was all they offered....the peel and stick floor tile seems to be working fine for now ..just the right thickness to hold mag in firm.(one piece on each side of holder). Will see what really hot weather does for it......I find that the glock mag carriers have some flex to them...how is retention once it's on your belt? It looks like they might tighten up once a belt is flexing them.
Also, worst case as mentioned, a buildup of a peel and stick material might be helpful, just check it in hot weather as I've had gummy stuff soften up and relocate (including skateboard tape). In pouches with a screw eyor similar running through them, I've made and bent thin kydex sheets to create removable retention, for example altering an XD45 pouch to fit M&P magazines, but I don't see a good way to modify the glock pouch in that way.