People able to maintain a lower body weight due to "fast metabolism" is mostly myth.
Because if someone is eating more calories than they are expending, they will gain weight. Speed of their metabolism notwithstanding.
Plenty of factors, cultural and genetic, at play.
Metabolism speed isnt junk science but in actuality it's a rounding error compared to the 3600kcals necessary to create one lb of body fat.
Have you been tested for genetic disorders? Specifically, cystic fibrosis?