This prompted me to refresh my memory on the dragons, and yeah, you are absolutely right. A few facts (none of which apply to Drogon, who is obviously just a kid, in Dragon terms).
1. as someone else mentioned, Balarion's fire was so hot it melted towers of Harrenhal.
2. the one time the three dragons took to the skies together, they burned 4,000 enemy soldiers. Drogon couldn't even do like 50.
3. Balarion's teeth were the size of swords, and his jaws wide enough that he could eat a mammoth whole.
By contrast, Drogon's teeth definitely aren't swords. See pic below.
One artist's image of Balarion, based on the description in teh books, makes him a bit bigger than Drogon. If you look carefully at the picture below, you'll notice tiny little Aegon the Conqueror on his back...
Quite different from.
