On a frontal chest radiograph the superior and posterior mediastinum are visible parts of the “cardiomediastinal” silhouette. (The superior mediastinal silhouette is composed of the vascular pedicle, and the posterior mediastinum is defined by the descending thoracic aorta on the left and azygoesophageal recess on the right.)

Saying the “cardiomediastinal silhouette is enlarged” indicates enlargement of not just the heart, but the other cardiomediastinal constituents too. If the goal is to describe cardiomegally, saying “the cardiac silhouette is enlarged” is more precise. Stated more concisely, “Enlarged cardiac silhouette.”