As I understand it, the current medical consensus is that fat protects muscle, and has health benefits when it is in moderation, but increases risks for bad outcomes when in excess. And muscle weighs more than fat, and aside from heart disease, generally protects against death of all causes. If muscle is generally good, and fat is good in moderation, why do we still popularly conflate skinniness as healthiness?
To expand on this, the original statistical basis for “normal” BMI is deeply flawed:
The History and Faults of the Body Mass Index and Where to Look Next: A Literature Review
The statistical averages derived from the foundational study of BMI really only have any meaning if you are a male with European ancestry - no other group was properly represented.