I think the specifics of your hypothetical math are kind of a non-issue in the real world.
I'm guessing you've never actually seen the bladder inside a scuba wing.
I have taken my DSS wing apart to look at before. It's an LCD 30, so it does have zippered access to the bladder.
You are looking at it two-dimensionally and getting the impression that a wing with double the capacity would look twice as big, but it won't. Essentially, it only needs to be 40% bigger too hold twice as much volume.
It looks like that Oxycheq bladder might be bigger than it really needs to be.
Using 17 and 35 in my example was an attempt to represent a somewhat extreme case. I would be very happy if my current 30 # wing could be zipped down to 20 # of lift. For that, instead of needing a bladder that's 40% oversized, you'd only need one that's (total guess on the square root of 1.5) 20-25% oversized.
---------- Post added January 20th, 2015 at 10:02 PM ----------
Stuart, I realize that you don't have to decrease size of the pontoon much to decrease the lift. But as I gave an example, even a bladder that was improperly installed so that it was somewhat crumpled can result in air-trapping. How are you going to decrease this size smoothly and not allow any spaces that are slow to vent? It's just so much easier to buy a different wing, if you really need different sizing.
As far as inner tubes go -- remember the pressure an inner tube is containing (and requires to be inflated). It would be very difficult to orally inflate an inner tube to its stretched volume, and oral inflation is a critical safety thing with BCs.
I don't have all the answers. I was just throwing out ideas. But, I have to wonder if the improperly installed bladder was having a problem because of the way it was installed and that a properly installed oversized bladder (which is scrunched up evenly all the way around - and which is not TOO much oversized) would avoid having the same issues.
I keep thinking about installing inner tubes in motorcycle wheels. If you don't partially inflate the tube when you're putting it in, it can get folded around or pinched and have a problem. But, if you do put some air in it as you install it, then everything goes together correctly. And then if you ever do completely deflate it, it will reinflate just fine because it collapses evenly all around. At that point, it won't collapse into a fold that will get trapped or pinched or trap air.
I suspect that a bladder of extra thick/stiff material might be even less prone to fold and trap air.
