Is it permissible to ask Allah for an increase in physical strength, and does that contradict being content with Allah's decree? And what is meant by the Hadith: "A strong believer is better and more beloved to Allah than a weak believer," and does it apply to being thin?

Thinness may be a disease or natural. Pathological thinness is treated with permissible things such as foods and herbs, and may require psychological treatment. It is permissible for a Muslim to seek treatment with the intention of strengthening oneself for obedience to Allah, and this does not contradict being content with Allah's decree. The Hadith, "A strong believer is better and more beloved to Allah than a weak believer," means strength in faith, knowledge, obedience, and strength of opinion and self. Physical strength is added to this if it aids in righteous deeds. Physical strength alone is not praiseworthy unless it is used in what pleases Allah. Thinness is not a blameworthy weakness if its possessor is active in obedience, and obesity is not strength if its possessor is lazy. If there is no disease, then praise be to Allah for good health, and it is permissible to strive to modify the body with permissible things and supplication.