When we do thoracic radiographs we are looking mainly at the heart and lungs. There are other things in the chest, but the heart (and blood vessels) and lungs are the most commonly examined areas.
On the top is a normal thorax with a normal-sized heart (this is a cat). On the bottom is an enlarged heart (dog). There are some differences between cats and dogs in general heart shape, but these two radiographs nicely illustrate the degree of heart enlargement we can see. Notice that the dog's heart seems relatively "wide". It occupies a fair amount of the chest cavity. If we measure across by rib spaces, it is almost 4 rib spaces wide compared to the cat's two.
This heart also touches the sternum a lot (we call this "sternal contact"). Lastly, look at the angle between the trachea and the spine. In a normal thorax the trachea and spine are at a 15 degree angle. In this dog, the trachea is running parallel to the spine. This is because the heart is enlarged and pushing up on the trachea.
When we look at the other view, with the animal lying on its back and arms in the air, we can also see the difference. The normal heart (cat) is on the left, abnormal (dog) on the right. The dog's heart has quite prominent bulges that the normal cat's heart lacks. The dog in this case was suffering from heart failure. Various chambers of the heart had enlarged and subsequently weakened. The heart had a hard time pumping the blood. With medication this dog lived for several years very happily.
This radiograph is a sad one. Notice that there is very little black compared to the other radiographs. This animal had very little air capacity in the lungs. The lungs are filled with many, many spherical densities that almost obliterate the lungs. This is a case of cancer in the lungs. These tumors had metastasized there from somewhere else in the body.