It's the baby's nickname, not the official name. I think it's something common in Asia to give newborns nicknames even before they are born(? I could be wrong tho.
but i think it's in the baby's birth certificate(?) i'm talking abt the paper at the last panel where the "feng big head" is written. and we don't write our nicknames on our birth certificate. the name that's written on ur birth certificate will be ur name for the rest of ur life. unless u want it to be changed. u will pay like over a thousand dollars to change it. so yea i think that's his official name.
if my parents named me big head, i would be cryin too. WHO WANTS TO BE NAMED BIG HEAD???? I LITERALLY SNORTED WHEN I SAW THE CHILD'S BIRTH CERTIFICATE (?) AND IT SAYS, FENG BIG HEAD. LMAO ROUROU WHY WOULD U NAME A PRECIOUS BEAN LIKE THAT! but it suits him tho.