Curiousv:
I wouldn't use weight alone, not even BMI, as people have different body proportions, fitness levels etc. regardless of weight.
I would use relative categories, mostly focused on their effects on daily life.
For 1 to 10 (I intentionally designed this system so that almost everyone fits into 1-6, ideal if you want to create fantasy scenarios with dice). I made this list with women in mind, but you can adjust them to men if you wanted
1: skin and bones, even those who prefer thin would call her too thin. Unhealthy and dangerous. Likely has an eating disorder.
2: thinner than average, but not clinically so.
3: most people would consider this to be normal, and strive to reach this weight if they differ from it.
4: chubby, considered above average weight by most people, but she can still hide the extra weight by carefully chosen clothing and camera angles.
5: definitely fat, and can no longer hide it. Still, the extra weight is not too limiting in most daily tasks and activities. If really attractive and has a great personality, might still be found desirable by those who aren't specifically into fatties (but they see her weight still as a major negative point)
6: obese, the weight starts to impose limits on many activities. People who aren't into fatties start feeling at least somewhat disgusted. Folds appear on different parts of the body regardless of posture, belly can firmly rest in the laps when sitting, some chairs become too tight, some difficulties when picking up things from the floor, basically the biggest size one can get without having either a severe eating disorder or being into feederism, and the biggest size most people will regularly see outside reality TV, or the Internet.
7. fatter than most things are designed for, cannot fit into small cars, into some restaurant booths, and even the largest clothes in supersize clothing stores start becoming too small. I consider the border betrween 6 and 7 to be the border between BBW and SSBBW.
8. still being able to care for themselves, but only with great difficulties. Standing up, or going up a single flight of stairs, or walking more than a few dozen steps, or just standing more then a few minutes are all very exhausting. Basically the biggest size one can have while being able to live completely independently.
9. requires a caretaker for everyday life, but is not completely bedridden. Still, getting up is difficult and might require help.
10. immobility.
Probably between 7 and 8 such a big fatty mmmmm
