So, one of the most common theories about how dragons could maybe still fly, the living gasbag theory.
Basically, most of the dragon is a cavity filled with lifting gas or air, in order to reduce its weight and density. Of course, this wouldn't make it even neutrally buoyant.
But, just how much of the dragon can be a glorified balloon without negatively affecting their other life functions and locomotion?
We're still going with the "195 cm tall at the withers, 2,95 meters at the top of the head in a relaxed posture, built like a horse, but the neck is elongated" type. There's also a sleek tail, no longer than the body, with a sharp bony structure (I think they call it a spade) at its end. And finally, a separate pair of wings, so 6 limbs in total.
I know that horses have tendons in their legs, and the actual muscle, sitting above, is connected to those, but I don't know how much I can abuse that.
Let's think about it this way:
We have Mini Smaug with the voulme (determined using water dispacement) n. You can't increase the volume, only decrease the amount of organic material there. How much could you remove before (see bold paragraph).