This is caused by the aesthetic trend. Most of the statues you see are from the Ming Dynasty. At that time, Buddhist statues always focused on complete peace, so they were fatter. If you appreciate the statues in the Tang Dynasty and the Northern Wei Dynasty, you will find that many statues are bony, because the aesthetics at that time were like that, while the earlier pre-Buddha statues unearthed in Peshawar were as thin as bones.