Gastroptosis
Gastroptosis is a medical condition where their internal organs are dislocated downwards, causing the stomach to protrude their belly. This condition is often prevalent on infants and toddlers in Elementary grades due to having weak abdominal muscles to support it.
Generally speaking, this results in an appearance where the person's waist is loose and their body becomes a square, flat-like build with a notably protruding belly. Combined with their flat build shape, their appearance is a reminiscent of a smooth squid, so they're often called "ikahara" (イカ腹, lit. squid belly) by combining gastroptosis and squid.
Though this means there is a protruding belly, it does not mean fat, so do not go on a diet without a doctor's supervision.
Loli Little girl Shota Boy's navel
Navel Shallow navel Outie navel
Big sister's belly...in a way, could be considered an antonym.