We estimated the heart and lung size of several species of small bats (Tadarida brasiliensis, Mormopterus kalinowski, Myotis chiloensis, Histiotus macrotus, H. montanus, Lasiurus borealis and L. cinereus) and compared these values to those of bats of larger size and other mammals. Our results confirmed that bats have the largest relative heart and lung size of all mammals. This is associated with the high energetic costs of flight. As expected, the mass-specific lung and heart sizes of small bats were larger than those of large bats. However, although relative heart mass decreased according to body mass, M_b^−0.21, lung volume was nearly isometric with body mass (exponent = 0.90). This exponent was close to unity, and between exponents reported previously (0.77 and 1.06). This suggests that small bats compensate the energetic cost of flight mainly by changes in cardiovascular morphology. The relative heart mass of both H. macrotus and H. montanus was particularly large, representing 1.71 and 2.18% of total body mass, respectively. These values correspond to 121.3 and 162.7%, respectively, of the expected values from allometric relationships. In these big-eared bats, the large hearts could be attributed to the energetic costs induced by the ears' drag.