Yes, that is exactly what I was going for. Start with a real creature; scale it up; figure out the hypothetical properties (weight and strength) that would be required to keep it airborne at that size; and then invoke magic to make it so.All of that is fine, but at some point magic needs to be involved. Dragons as we know them could not exist without magic. That doesn't mean that like beholders they can only fly because of magic, just that they are magically lighter than their mass would indicate.
To me that makes sense. If there is magic, evolution will at least in some cases take advantage of it. So a dragon (at least while flying) weighs as much as a very large bird and it's bones are supernaturally strong. Of course it's scales are also supernaturally strong and it breaths fire so it's just part of the package.