Assuming it's the same as my 18l - the bag gets looser as the water in the bladder goes down. Never found it a problem - just take up the slack as you get through your drink! (Never needed to stop for it!)
I've just bought the OMM Ultra 4L. Used it once, today, and it seems pretty good... but it is probably just a little bit too big for me. Even wearing it quite low down over my hips I have to fasten it as tight as it will go to stop it bouncing around. The minimum size seems to be about 32".
I'm sure there must be other runners out there who would have the same problem, it can't just be me. And it seemed OK in the shop
I saw a few people with the new OMM bumbags at the weekend. They do seem VERY big.
Can imagine they are good when used in conjunction with the Last drop rucksack and if you got very little kit that would be enough for a mountain marathon but as a stand alone item they will just be prone to bouncing around.
Inov8 are releasing some more bumbags and rucksacks soon by the way.
Playing with one now! check out http://middlepack.co.uk as there is some piccies there.
i like those new inov8 pacs, but i do prefer the blindingly obvious yellow,