My dog's heart got too big. What heart disease caused his death?

I am so sorry to hear about your loss of sweet Hotdog. It sounds like he likely suffered from heart disease. There are a few different types of heart diseases that dogs commonly get like mitral valve insufficiency, dilated cardiomyopathy, and right sided heart failure (common with heartworm disease). Unfortunately since I don't have the full history and I didn't do an exam on him I cannot say for sure which of these he had or if it was a different type of heart disease. Most heart disease eventually causes congestive heart failure which is where the heart is not strong enough to pump the blood forward through the body properly and therefore it begins to back up into the heart and eventually into the lungs causing fluid in the lungs. If it was right sided heart failure the fluid could back up to be around the lungs and into the abdomen. Most dogs can compensate for this for awhile without the owners ever knowing there is a problem, but eventually they can no longer properly compensate, begin showing symptoms and pass away. A lot of times by the time you see symptoms, the heart disease is so advanced and there is no amount of medicine that can help them. Heart disease is such a hard thing to deal with and I am so sorry that poor Hotdog had this. There is nothing you could have done differently to prevent this from progressing. Hotdog knows you loved him and was lucky to have such a wonderful family. Please know you are in our thoughts during this difficult time.