I'm sorry to hear about the trouble with your bunny.
How big is the mass? Is it actually *in* the lungs or just overlapping/compressing the black (=airy) area of the lungs in the X-ray?
One possibility is a thymoma or a similar mass in the chest cavity. An examination with ultrasound could probably tell you more. An ultrasound can distinguish between fluid-filled cysts, abscesses, tumours, for example, and show the size, shape, and location better.
My own bunny was recently diagnosed with a large chest mass, most likely a thymoma. First, he had an X-ray, then an ultrasound (both done awake), and although he has no breathing problems (a common sign of thymoma), his lung capacity in the images is compromised. The only symptom we saw was that his third eyelids protruded, and his eyes bulged, just a little when he was eating pellets with his head down. Long story short, he's now on prednisolone for life (he's 10), and we're hoping that will at least slow the growth.