You have to account not necessarily for smoke or smoke content lost but THC destroyed by burning (which is what helps give vaporizers that boost in THC/tar ratio almost as much as the fact that less tar-able substance is burning). In a cigarette at least 90% of the nicotine is destroyed by burning, which is actually the reason that a few cigarettes in a row doesn't give you what would be, ingested, a dangerous and potentially fatal nicotine overdose. I don't have the exact percentage of THC destroyed by smoking, and I know it's not as close, but it's up there too. Most of the bad chemicals in smoke are the result of things burning... most of smoke, as a result, is composed just of chemicals that result from burning. A lot of the original stuff that can't burn well gets left as ash.

So now consider how much weed gets lit at all during a single hit from each device. Once the flame touches it, or worse just the heat, and even if it just browns it (heat but no burning), the THC heat damage has been done. All the THC that would have rode the smoke particles, assuming that little portion caught fire instead of just turning brown, is still there, ready to try again. Whether it'll survive the eventual lighting, when even more THC is destroyed, is an open question.

Pipes you relight, bongs you relight, and both can burn on their own for a variable period while not being hit from, during which time a large amount of the contents are exposed to close-by heat. In a joint only the end burns and the tubular structure keeps most of the heat going exclusively up and away from the remaining weed. Even though a joint is burning continuously, moreso than a bowl of any sort would, it gives off its heat the best. That whole metal bowl on your pipe gets up to 130F or higher at times and it just puts the THC that much closer to the temperature it has to hit to destabilize, at which it could break down. The joint remains tolerably cool enough to touch, even down to a small roach where the cherry is effectively millimeters from your hand. Think about it.

The remaining inefficiency that puts the bong below the pipe is in the THC lost to water. I can't see much being lost on the insides of the device because glass is, molecularly, smoother than the metal in a pipe. (A joint is the least smoothest but collected up resin is then smoked.) A glass pipe would narrowly outperform a metal pipe. But the bong does lose something in water, and probably also because its hard to inhale the whole hit and leave absolutely no smoke behind... also consider that the bigger the hit, the less of its total content gets deep into your lungs. If your hit ends because you've reached inhaling capacity, then necessarily the last of what you inhaled is still in your throat and not your lungs. This situation sometimes comes up in a bong hit, moreso than with, say, a pipe. Finally, measure and see that although the few big hits of a bong get you high, and possibly higher than you personally get with many small hits, they do in fact use more weed than conservatively-used pipes or thinly-rolled joints. Perhaps just as a whisky drunk is different than a beer drunk, the faster high of a bong from its large hits is a different type of high than the slower one, and more enjoyable to some people.