Yeah you're right. It will adapt to the size and shape of the outer bag. The problem is that the outer bag material is fairly thin nylon so wrinkles, overlaps and twists show up horribly. I'm not concerned about "wrinkles in my stylin'" but the wrinkles cause stress cracks. The trick is to get it not to big and not to small, just right. I'll probably end up throwing the first one away before I get it right.
Once fitted right, it's amazing how much pressure they can hold. We used to do a demonstration for new students. Two balloons in pillow cases brought up from sixty or seventy feet. Both balloons started out at as close to the same size as we could get them blowing them up. (Hint: The "Blower Uppers" looked silly trying to blow the things up and hang onto them at the same time. Plenty of the balloons broke free and took off for the surface in uncontrolled ascents) Both are put inside pillow cases.
One pillow case is left loose at the end. The other is tied close to the inflated balloon. Then both are held onto as we surfaced. The one inside the loose pillow case expanded until it popped. The one inside the tied and restricted pillow case was tight against the pillow case but it couldn't expand. The pillow case would blow out and the balloon would squirt out the hole and blow up.
A BC, even one of the old Horse Collars can take quite a bit of internal pressure because the air bladder is restricted by the outer cover. Of course, "quite a bit of pressure" doesn't include multiple atmospheres so the BC will blow up without over pressure valves.
