People have different bone sizes. This is a scientific fact. Finger length, leg length, head shape, pelvic width, size/shape of rib cage--all vary person to person. Even the size and shape of knee caps and other sesamoid bones are different in different bodies.
The common ratio that quantifies this difference is wrist circumference divided by height. At my most thin, a BMI of 24, I could not wear standard watches and had to get some dress shirts let out because I am genetically big boned.
On some people, this proportion looks "good". On others, even at an ideal weight, the person looks "stocky" or "thick". It is not considered a cause of obesity, but big-boned people are heavier than their narrow-wristed fellows. It's a simple matter of mass ratios.
Now, what goes into a person appearing "fit" or "pleasing" or "castable" or "sexually attractive" or "able to move" or "douchy" are matters of much subjective opinion.