SEOUL, South Korea—The health of North Korean leader Kim Jong Un has long been a source of morbid fascination in rival South Korea, which sits in the shadow of Kim’s 1.2-million-strong army and his growing arsenal of nuclear-armed missiles. Has he gained even more weight? Is he struggling for breath after relatively short walks? What about that cane? Why did he miss that important state anniversary? Now, the 37-year-old faces fresh speculation in the South about his health again. But this time, it’s because he’s noticeably slimmer. Kim’s health matters in Seoul, Washington, Tokyo, and other world capitals because he hasn’t publicly anointed a successor who would control an advancing nuclear program targeting the United States and its allies—if he is incapacitated. North Korea, never open about the internal workings of its leadership, has over the last year shut itself up even tighter to protect against the coronavirus pandemic. In …