Are they eating via other sources (Arrange Events dinner)? Anyway, weight loss has a small chance of occurring (-1kg) every midnight calculations. At poor rations (1R), they can go as low as 45kg.
As far as I can tell, this is also counter-acted by the effect of high Strength, which increases Weight.
- Very Petite Frame at STR 60+, chance to gain +1 weight up to 59kg
- Petite Frame at STR 70+, chance to gain +1 weight up to 64kg
- Normal Frame at STR 74+, chance to gain +1 weight up to 69kg
Guys of course cannot become a different body type. They are stuck with the body type they started with. The only exceptions are children since boys below 16(?) have Petite body type that transforms to Normal after reaching 16yo.