Haha Kokanee, I too have the Condor
. Popular piece of kit I guess...
For Rolly Polly's, the medium is the best size to get. The large one is a friggin laundry bag size and doesn't fold up nice and small.
As for keeping mags in, it's pretty dang good at that. There's the elastic drawstring you can tighten... I have mine a bit tight all the time so mags go in easy, but don't come out easy. Plus, the velcro lid that keeps it all rolled up when it's folded away, also is a lid for the bag when it's unfolded, so use the lid in conjunction that with the drawstring being partially tightened and it will do a wonderful job of keeping mags in.
I have my Rolly Polly on my CIRAS so I like to keep it folded up as much as possible to keep a lower profile for when I go prone and to keep it from inhibiting the full range of motion of my arms. So I use my Eagle SAS dropleg pouch primarily, then I unfold the Rolly Polly when that guy fills up.