To measure body fat rate, abdominal lines should look good, first of all, body fat should be low. If your stomach is small, you should first consider the role of fat (especially visceral fat), which is difficult to reduce and is often the last fat accumulation consumed by the body.

Secondly, we should consider the role of transverse abdominis muscle and oblique abdominis muscle. Transverse abdominis muscle and oblique abdominis muscle are mainly trained by plate support and its movement variants (such as side plates). It is not enough to strengthen the rectus abdominis.

By the way, if it's a woman, there will be exceptions, though few. Just do a gynecological examination.