स्थौल्य कर्ष्ये वरं कर्स्यं समुपकर्णौ हि तौ।
यध्युभौ व्याधिरागछेत स्थौलमेवतिपीडयेत ।।
if the same person, gets into similar disease condition, with obesity and underweight, the person would always be more harassed when obese. So, to make a choice, remaining underweight is always preferable.
Ayurveda is a life science. It has quite subtle, unique, basic (near to nature or developmental factors) and different thought process than contemporary sciences today. Why because it has begin with the origin of life. More to add, I think it has not been conceptualized or documented or researched – it’s a life science that’s already present from the beginning of existence.
Humanity has always preferred being more active and light, in all manner. Meaning that lightness of the body has always been preferred infront of heaviness. This is because the flow of life itself as चित्त, is always more adorable with lightness. Even at many places lightness is compared to knowledge and heaviness to Ignorance, in spiritual sciences.