I never made weight for my height while in the Army, but it was obvious I wasn’t overweight. At six foot and 225 pounds, I was over my max weight by 35 pounds — even though I crushed my PT tests.

Some countries, such as the U.S. and the U.K., have seen the average height of their population level off over the last several decades—the U.S. has dropped rather precipitously in the world ...