Genetics is the number one cause. Here, it cannot even be explained as a matter of "bloodline" or lineage, as fish exhibiting a pronounced bulging belly typically do not belong to well-bred, high-grade (bloodline) stock. In non-specialized or less professional koi farms, breeders often select broodstock with larger abdomens in pursuit of higher yield (more eggs), leading to the propagation of this trait across generations and resulting in a high proportion of offspring with bulging bellies.
There is also another common cause: during the ovarian development stage of female koi, typically at water temperatures between 18–22°C, excessive feeding can lead to abnormal abdominal enlargement, contributing to this undesirable body shape.
