I suppose you are driving very fast right now and your pleasure at this activity inpires you to mimicry. (Please be careful.)
It’s a fantasy world and the only rule is the pen of the script writer. Fire immunity is not a thing, living creatures can’t breathe fire (hey, physics moron me), so anything goes. Settled.
They cancome damn close, though…
I thought for aerodynamics, more atmosphere always makes it easier to fly.
No. For something the size of Drogon to fly, even assuming hollow bones and air sacs, the air would have to be so thick that people would be moving around as if they were walking under water. Finagling the gravity and air density enough to allow Drogon to fly would cause everything else in the world to move drastically differently than it does.
To reiterate, Drogon is 20 times larger than the largest flying animal known on earth, and we don’t even know how that flew. Other dragons were even larger than Drogon is now.
Very large creatures can “fly” in water, but not in a much less dense medium.
Actually, figuring out how to get an animal to breathe fire would be a lot easier than giving an animal the size of a dragon the ability to fly under the apparent gravitational and atmospheric conditions of the GoT world. After all, many Earth organisms produce flammable hydrocarbons and alcohols. You just need an exothermic reaction (like the one in MrDibble’s link) that can produce a high enough temperature for the fuel to ignite.
Given the way the dragons interact with solid wood and stone objects, I’m pretty sure their bones aren’t hollow. Those sacs must be full of magic buoyancy fluid.
On how Balerion and Drogon are able to fly, my go to answer for these settings is - A wizard did it.
Thoughts on Jon Snow as DAKINGINDANORF; next season we really need to see him preparing for the *real *war to come. He warned whats-his-face Cerwyn that the enemy won’t wait out the storm, he brings the storm. Now that the human villains (with the exception of Cersei, who I think will be killed by Jaime in Season 7, Episode 9) have largely been dealt with let’s see Jon order in stockpiles of pitch, Dragonglass and Valyrian Steel, reinforce the Night’s Watch as the likely first point of contact for the Night King’s invasion.
Maybe I missed this part of the discussion, but how is Drogon 20 times bigger than a quetzalcoatlus (or whatever the largest pterosaur was)? Weren’t those things huge, if not quite as big as Drogon?
Aside: Yes, this is a silly discussion, but I find it fun. 
Here you go.
Even if you gave Drogon hollow bones, he’d still weigh far more than a Quetzalcoatlus.
Okay… but he’s not nearly as bulky as a T-Rex. If we can assume carbon nanotube lattice bones…
I get your point. Just trying to find a finger of a sliver of a chance to suspend my disbelief on. As it is I’ll pretend that GRRM and HBO are just using hyperbole for the sake of fun and drama, while the “real” version of the story had smaller dragons.
Those bones aren’t just hollow, but filled with hydrogen, which is also stored in large sacks throughout the body; that’s also what they use for breathing fire.
Well, or maybe not… Although it would add an extra element of danger to the dragons if they were also highly explosive. 
Besides dragons, we have armies made up of the living dead (some hardly more than skeletons), assassins made out of smoke, mind-control of animals and people, time-travel, etc, etc, etc. Dragons seem like one of more believable things if you’re going to suspend your disbelief. I mean, what kind of physics do smoke-assassins and telepathy depend on?
Heh. I can just see Drogon being hit with a fire arrow in one of his hydrogen sacs and going down in flames like the Hindenburg. 
And yet, any explanation that tries to change the world around him (thicker/different atmosphere, lower gravity etc.), is soon chock-full with unintended consequences.
Personally, I find that people acting in ways inconsistent with their established characters, or doing obviously stupid things take me out of it far more than any of the supernatural elements like dragons. I mean, the fantastical elements by their nature can’t be expected to be strictly realistic. But people are still people, and when they act unrealistically it’s harder to accept.
Yes. We do know that Dany, Tyrion, Jon and Sansa are as unreal as a dragon, they are all lies; but in contrast to dragons, they are a representation of something we are familiar with, and so there is an expectation of conformity to known rules to make them human in our mind, so that we can care. If they fail, they fall into “uncanny valley”-territory, which is as irritating as it is frustrating.
Just because the inhabitants of Planetos look like humans doesn’t mean we can assume that they are. In fact, there’s no way they could be, since it’s not Earth. They could have vastly different brain structures, neural pathways, chemical balances of dopamine and seratonin and all those reward systems, etc. As alien creatures, I’m sure all those decisions make perfect sense to them. 
Well, they are neither alien nor creatures, but inventions. 
And their very human creators make no effort to distinguish them from us but present them as human: they look like us, work like us, walk and talk like us, and they live in a world that is made up of building blocks that are familiar to us, as part of history or our cultural mindscape.
Any difference we have seen so far seems to be related to the one element not present in our reality but well known to us as a concept: magic.
And since the existence of the supernatural is so deeply ingrained in our psyche that the ones who don’t believe in it at all are the minority, not even the reality of magic is necessarily alien to everyone in the audience.
Of course, the show has a more difficult task than a writer to show magic in a plausible way: when we read about an attack of three dragons against a slaver ship, we create the images on our own, which makes them plausible.
Watching the images created by someone else, adds mental distance, and gaps appear.
Dragon sex. It’s hawt!