平胸随我爸胸大随我妈
[píng xiōng suí wŏ bà xiōng dà suí wŏ mā]
This username plays on a Chinese phrase that expresses whether one inherits physical traits from either the father or mother, here specifically about breast size. 'Flat-chested from Dad, big-chested from Mom'. It humorously addresses genetic inheritance in a playful manner.