Fat transfer can sometimes be used to improve contour in selected areas of the thighs, but using it to significantly augment the quadriceps or hamstrings is not a routine or highly predictable procedure. The legs are mobile, muscular areas, and the amount of fat that survives can vary. A modest contour improvement may be possible, but it is usually not a reliable way to create true muscle-like enlargement. Safety depends heavily on where the fat is placed. In general, fat grafting should be kept in safe soft-tissue planes and not injected into or around major vessels, nerves, or deep muscle compartments. Potential issues include unevenness, lumps, fat necrosis, oil cysts, asymmetry, infection, and the need for more than one procedure. There is also a limit based on how much donor fat you have and how much the tissues can safely accept. I would recommend an in-person consultation with a board-certified plastic surgeon who has specific experience with lower-body contouring. Bring goal photos and ask what degree of change is realistic, where the fat would be placed, and what alternatives may be safer or more predictable for your goals.