Weight is measured by the Sim's Fit and Fat. Fit being their muscle or the ability to complete endurance exercises and not their Fitness skill. Similar to how a scale would measure a Human's Muscle and Fat.

 

The range for Fit and Fat is set according to the game's thresholds:

 

HIGH: anything over 30% Fit/Fat   |   LOW: anything less than 30%

 

It is prudent to watch your Sim's Weight since this mod does add in tendencies to lose Energy (10%), reduce a Sim's lifespan (.85x) or affect other Health Vital Points. On the other hand, if your Sim's Weight measurement comes back Fit, it can also increase Energy (10%) and increase a Sim's lifespan (1.20x).​