Midrash for Chullin 99:34

א"ל

The law, however, is that the defect in one lung may be compared with the defect in the other lung, the small<span class="x" onmousemove="('comment',' I.e., the defect in the lungs of a small animal, e.g. sheep or goat, with the defect in the lungs of another small animal: so R. Hananel and first explanation of Rashi. Another suggestion in Rashi is: the defect in the main lobe of one lung with the defect in the main lobe of the other lung, and the defect in the small lobes of one lung with the defect in the small lobes of the other lung.');">¹⁷</span>

Sifrei Devarim