Oh, I don't know... I can think of three options that come pretty close in size.

Probably the best is the fish transport bags that David Chinell (dchinell) mentioned in his recent post. I still have several, and they are very flat and thin indeed.

Reynolds used to make their "oven bags" in medium and large as well as the big turkey bags that they sell now. I have a couple of boxes of the old medium size, and I've carried one in my wallet for a long time now. Don't even know it's there. As I recall, it was about a quart and a half.

I can tell you from my personal testing that both are amazingly tough in use. None of them will withstand sharp objects, but the bags withstood dropping, full, onto a flat surface much better than the condoms. Of course, in the forest, there will always be a twig...

You can always use a latex balloon, of course... they're compact and cheap, and thicker than a condom, but in the wild, it's much, much harder to fill than a condom or either of the bags. The condum will expand from just the weight of the water that's filling it much easier than a balloon will.

The argument is sometimes made that the bags can't easily be tied shut, and need a twist-tie or something, whereas you can tie the balloon or condom. This seems valid until you try to untie them to drink...

It's worth noting that exposure to heat will deteriorate latex condoms, the more heat the faster. When that happens, they just split as soon as you try to use them- not good. Those who store them in wallets and glove compartments often find this out at awkward times.