The BMI definition of "overweight" is pretty tight compared to general American sensibilities. It leaves a lot of room for being cosmetically overweight rather than being a significant health issue and yeah, unusually fit or broadly built people will often be caught in that category as well. It's also just the nature of weight that it's much easier to be over and to be a significant outlier far, FAR over than under. The distribution is right skewed.

That said, North Americans are absolutely majority overweight and have been so long enough that we have a warped perception of weight compared to many places where this is not a decades old issue.