There's not even a comparison.
And the difference between Smaug and Ancalagon is not just in size, but also in power.
Smaug was of course a mighty beast capable of destroying the realms of Erebor, Dale, and Esgaroth, and it would certainly have incinerated Lothlórien and Rivendell if used by Sauron — it was even rumored that he caused earthquakes in the region around the Mountain, which he could shaken. He was the biggest dragon of his time and he was really very big, old, and smart.
“One moment!” I said. “You hope to deal with a Dragon; and he is not only very great, but he is now also old and very cunning. From the beginning of your adventure you must allow for this: his memory, and his sense of smell.” (1)
His fall destroyed the whole town of Esgaroth:
With a shriek that deafened men, felled trees and split stone, Smaug shot spouting into the air, turned over and crashed down from on high in ruin. Full on the town he fell. His last throes splintered it to sparks and gledes. The lake roared in. A vast steam leaped up, white in the sudden dark under the moon. There was a hiss, a gushing whirl, and then silence. And that was the end of Smaug and Esgaroth, but not of Bard. (2)
Smaug Destroys Esgaroth by Gaius31duke
Esgaroth was neither a village like Bree nor a great city like Minas Tirith. It was a town like Edoras. Of course, Smaug was not the size of a village, let alone a town, he had only enough impact and power to destroy the town.
The same goes for Ancalagon, with one crucial difference:
Thangorodrim was HUGE and it wasn't one mountain like Erebor but three peaks.
Ancalagon must have been gigantic for its impact to have touched the three peaks. We're talking about a titanic dragon indeed that would make Smaug look like a lizard, for no one in right mind would say that Smaug could level Erebor with his fall, and that was a small mountain compared to a peak of Thangorodrim.
Ancalagon was also very powerful. He and his fleet, which was composed of another hundred winged dragons—of lesser size and power than Ancalagon—made the Host of the Valar retreat, and it is said that the reason for this was because their coming caused storms of lightning and fire. After this, It took Eärendil on Vingilot with a Silmaril and a host of eagles to defeat Ancalagon, twenty-four hours of battle, when already dead he fell on Thangodrim.
Now you may say that Ancalagon was BIG enough to reach the triple range of Thangorodrim, but you also have to note how POWERFUL it was and what impact it caused, and if you think Ancalagon merely leveled three mountains taller than Everest think again.
Two things destroyed Beleriand, the small subcontinent of Western Middle-earth: the fury of the adversaries in the Great Battle and the tumults of the breaking of Thangorodrim, which was caused by Ancalagon itself.
In the Great Battle and the tumults of the fall of Thangorodrim there were mighty convulsions in the earth, and Beleriand was broken and laid waste; and northward and westward many lands sank beneath the waters of the Great Sea. (3)
Beleriand and Eriador by Didier Willis / Ancalagon the Black by DoomGuy26
In conclusion, Smaug was only a powerful and big Dragon for his time. Ancalagon was a Beast imbued with the power of Morgoth, whose fall contributed to the collapse of a large part of Middle-earth rather than a large wooden town.
Sources:
(1) Unfinished Tales: of Númenor and Middle-earth
(2) The Hobbit
(3) The Silmarillion