First, my apologies if my response set the wrong tone! My point was simply that how big a dragon is depends on the game/source material, as does how hot its breath might be.
But, as for your sources they are just that--a couple variants/options. For the 5E DMG, a 10x10x10 hunk of solid rock
might average 27 hp, but could have up to 50, and would be resistance to fire, doubling it to up to 100, which an ancient Red Dragon might be able to destroy...

But you don't really know because 5E is too abstract. How does the AC fit in, combined with the hp? I can't assume every large object made of wood, stone, iron, and adamantine
all average 27 hp... I suppose I
could, but that is very unrealistic IMO.
For Game of Thrones---never saw it, but I'll take your word on it.
Finally, magic does "break reality" as it is meant to do. And "magic" armor probably wouldn't melt, although you could be roasted alive inside it... Also, my goal isn't to make D&D more realistic, but to find a game which already does so. If I did want to just keep hacking away at D&D, I would agree most people would probably get something like the CON in hp and never a point more.