I've noticed, after taking a huge bong rip, that the mass of exhaled smoke looks disproportional to the inhaled smoke. I'm so high right now, but I swear that's how it happens, especially with gravity bongs. I'll rip a whole bowl on the a gravity bong, only to blow out what appears to be a small amount of smoke. I know I'm taking big rips because I am rendered retarded every time.
It loses more of it's colour the longer you hold it in. That's why holding in a hit for a long time is called "ghosting" a hit, because when you exhale it comes out mostly clear. So since you hold in the smoke from a gravity bong longer, it looks like less comes out even though you've just taken a huge rip.
That's the thing - it still appears to be much thinner, even though I blow it out almost instantly. I've taken much smaller rips from bowls, even after holding it in for at least 7 seconds.
The smoke absorbed into your lungs and has entered your bloodstream on it's way to the brain. You should be greatful.
It could also be the fact that the water in the bong is filtering the smoke, so there's less impurities, which could translate into thinner smoke than straight hits from bowls.
My guess was just that the larger chamber = more air too. With a pipe, you're inhaling the bit of air in there at the beginning and from then on it's all smoke. With a bong, you're inhaling that large chamber full of air and then the smoke once you finish milking it. I also wonder if the smoke absorbs into your lungs faster after a deep bong hit rather than a more "shallow" feeling hit from a pipe.