What does that even mean? These terms are just descriptors. Some people are skinny and have a hard time putting on weight. Some people are large and put on weight very easily. Other people are of average build and their body's respond in a manner somewhere in between the other two. That's all those terms mean.
Look up William Herbert Sheldon sometime.
Frankly, it astonishes me that people would use those terms yet be totally oblivious to how they originated and the quack science that surrounds them.