Exophthalmos is the most commonly measured sign of Graves ophthalmopathy, whereas enlargement of the extraocular muscles is the principal pathologic abnormality. The purpose of this article is to explore possible etiologies of increased volume of orbital fat and of proptosis in patients with no substantially increased total extraocular muscle volume. Computed tomographic scans of the 13 orbits reviewed in this study had the following characteristics in common: a fine, reticular pattern within the orbital fat, a prominent superior ophthalmic vein, and an enlarged superior rectus muscle. Quantitative analysis revealed that superior rectus muscle volume showed a statistically significant correlation with proptosis, whereas medial, lateral, and inferior rectus muscle volumes did not correlate with proptosis. Based on anatomic considerations, the authors postulate that superior rectus muscle enlargement alone may produce reduced venous outflow from the orbit, thereby expanding the apparent orbital fat volume and producing proptosis.