I'm not disputing the thresholds you're proposing, but weight is more of a sliding scale than the sort of well defined tiers you're listing. If gaining 1 pound moves you from 29.9 BMI to 30 BMI, you didn't suddenly go from slightly overweight to obese. Part of it depends on genetics, height, etc as well. To me a better measure is whether they have visible abs, how well defined they are, and if not, how much weight they would need to lose to have visible abs. That kind of takes height and musculature out of the picture. So a girl who's a gym rat and is muscular (for a girl) could have significantly higher BMI than a girl who does not work out at all and still look slimmer. Same for guys.Even if we set aside the unreliability of BMI as a means of assessing your health or even how fat you are, 29 is 'merely' overweight, one becomes obese with a BMI of 30+
The ranges are Underweight >18.5, Normal Weight 18.5 - 24.9, Overweight 25-29.9 and as said earlier obese which is 30+
If we are going to use these sort of measures we may as well use them right. Now how this directly corelates into gamescales or how, if at all, it should be implemented in gameplay is another question entirely but we might as well make sure everyone is clear on what BMI measures actually are first.
Musculature is kind of a separate metric. You could take a marathon runner and a football player or body builder with the same body fat percentage (not BMI) and they would have wildly different physiques.