"Overweight" in this context doesn't have much to do with clothing sizes. It's a quasi-medical definition that is based on the BMI (a ratio of height to weight). Various medical organizations eventually landed on the idea that a BMI of 25-30 is "overweight," but this was a pretty arbitrary definition and doesn't mean much. Research on the links between BMI and health outcomes have not produced evidence that BMIs in the "overweight" range are associated with poor health, and there's some evidence that they can protect people from diseases that sap your energy. On top of this, BMI does not distinguish between men and women, despite the fact that women are naturally heavier for a given height.

The question of what should be considered "medium" or "normal" in clothing sizes is pretty separate. Different brands have completely different sizing schemes, and many are intentionally set up to flatter their buyers or for some other purpose than clearly communicating the dimensions of the clothing.