I love the credit people are giving Varric. Surely what we're seeing is Cassandras vision of the story as being told by Varric?
The same way people have an image of King Arthur in their heads, or Jesus, or Saladin. We have an expectation of certain heroes and villains in our heads, and we're rarely accurate. Especially not when all we have is fables of the legend behind the man. Thats why movie adaptations are always put under fire by fans who always saw the Main hero/villain/side-character as something different to the movies end result.
If the tale Varric is weaving is going to flow at all, it'll be a rather swift descriptiong of hawke to get him to the action. You know the sort of thing.
"The hero of our tale, the dashingly rugged hero of Kirkwall, the champion of the land, the mighty Hawke, set off on his journey with his Sister, the valiant and well endowed beauty know as Bethany."
So far, all entirely subjective, with no descriptions. But, because some people are visual, they're setting an image in their heads of these two. Hawke is Cassandras idea of ruggedly handsome. Bearded, youthful but fierce, blindingly bright eyes, a sure swagger in his step. Bethany is a comely lass, but busty and kind, because she's the sister of the hero. She doesn't need to be a blonde bombshell, indeed it'd be rather against the archetype, so she isn't.
When Cassandra finally calls him on the ridiculousness of his tale, varric smiles slyly and them explains
"In truth, Hawke was never the bearded vengeful templar hewn from the stone. He was a blonde haired rapscallion, a joker even when it would be irresponsible, a nervous young thing with dark eyes and a rather unappealing face. Legends will run away with themselves though, and the people pictured his as this bronzed god. A pity too" etc etc.
Thats how legends work. Chinese whispers, to the point where the legend is more well known than the truth. You say that "yes, but that take thousands of years" but I'm going to say just two words. Chuck norris. Memetic badass, and certainly far more famous amongst some for the "Chuck norris facts" website than his string of cheesy movies or short-lived TV show, or indeed his Karate Career.
Now imagine, instead of TV, the internet, dvds and other methods of communication, all you have to go on is those facts, as told by a local bard. and you live in a world where Ogres roam free, Dragons exist and magic is fairly common place. Would all of those facts be so hard to believe now? And would you believe they are about a man now over 60, with grey hair and a wrinkeled face, or would you picture him as a 12 foot tall bronzed god with pecs of iron and muscles of pure steel?