In The Silmarillion, Tolkien himself acknowledges he makes some embellishments and exaggerations of his descriptions. There is an INCREDIBLY detailed breakdown of the size problem of Ancalagon the Black if you are interested reading at….
A visual comparison between Smaug and Ancalagon the Black from the Tolkien universe, showing their massive size difference. Smaug is depicted as a large, fearsome dragon perched on a mountain, surrounded by gold and treasure. Ancalagon the Black, much larger and more colossal, is shown towering above ...