Wow, that is a big heart.
Did your vet perform a cardiac ultrasound, or just the xrays? I would strongly recommend an ultrasound to get an accurate diagnosis - with that heart shape on xrays, I couldn't be sure if it is a DCM or fluid around the heart. An ultrasound would let you know for sure what you are dealing with.
If it is DCM, I would certainly look at medication. Pimobendan would be my first choice as it helps the heart contract, plus makes it easier for the heart to pump blood. If there are signs of abnormal heart rhythms, it may be worth using other drugs such as digoxin. Fluid tablets might also be useful.