This is why the thong dissappear

A larger or fuller butt causes thongs to be more fully enveloped by the buttocks. This depends entirely on the body’s anatomy and how the fabric drapes between the muscles and fatty tissue.
• Natural depth: A larger butt has more volume in the gluteal muscles (gluteus maximus) and fatty tissue. This makes the natural crease between the buttocks deeper, causing the thong’s back strap (the string) to sit further in and be more concealed.