The phrase "great dragon" refers to a powerful and enormous mythical creature that is often depicted as a large reptile with wings and the ability to breathe fire. It symbolizes strength, wisdom, and sometimes evil or danger. Full definition
There are a number of great dragon paintings from ancient China, like the Nine Dragons (1244) at the Boston MFA. (brooklynrail.org)
Once he's awake, Smaug certainly is a flaming great dragon and you quickly forget it's Benedict Cumberbatch on vocals and that it's Weta Digital on everything else. (nzherald.co.nz)
Of course, The Desolation Of Smaug's cliffhanger ending left the leader of the Company of Dwarves in a state of increasing agitation, with the pressures of his quest and that massive great dragon hellbent on flaming destruction weighing heavily. (empireonline.com)