>>18852>they say that the lighter you weight the longer and better you live because something about the human bodyThat's probably related to the common confusion between losing weight and losing fat.
>muscles weigh more than fatPer same volume, meaning they don't simply weigh more (which by itself makes no sense), they are more dense. So your argument would only apply to volume i.e. same volume of fat would according to the above logic be healthier (weigh less) than same volume of muscle. But fat definitely poses a higher risk to health than muscle lol. Of course extremely low fat is also unhealthy.
There still might be some truth to lower muscle mass being healthier to some degree, if you compare a gorilla mode to an otter mode: less energy needs to be consumed, so less food needs to be processed, so less stress is put on body's various organs.
Skinny sticc on the other hand will be dangerous for your joints among other things, not to mention your body being barely functional in everyday life. Otter mode is definitely better.