Sort-of memes that are cracking you up at the moment

The problem with the meme is that it significantly underestimates the amount of gold Smaug is sitting on

Agreed.

"An entire mountain full of almost nothing but solid gold" would have to be at least 1000 times as much as that.

Surprisingly, not quite that much. Since you enjoy a good technicality, Mount Wycheproof in Australia, the world’s smallest registered mountain, stands at 43m above the surrounding countryside. It’s not clear whether it’s a conical frustrum or more of a sphere cap, but by guesstimating its radius based on Google pics showing its height (*1.5r:1h), I reckon it has a volume of some 300,000m^3. Still much more than the Forbes estimate, but encouragingly less than 1 mill m^3. I have no idea what the canonical volume of the Lonely Mountain would be, and we don’t know enough about dwarven architecture and mining techniques to know how much of it would be solid rock.