Antonyms for Pyknic (opposite meaning)
WordNet sense 1 (having a robust muscular body-build characterized by predominance of structures (bone and muscle and connective tissue) developed from the embryonic mesodermal layer):
mesomorphicWordNet sense 2 (having a build with little fat or muscle but with long limbs):
ectomorphicFrom the ODE community, based on WordNetadd/edit