Mega Fat CEO Baby used to be Mega Fat Mail Room Intern Baby, but gave the Boss Baby an idea he thought was spectacular: babies delivered in giant envelopes. Boss Baby thought he was brilliant and promoted him to his assistant.
Mega Fat CEO Baby was a good friend of Boss Baby, when he was Mega Fat Intern Baby, but, Boss Baby betrayed him. Boss Baby used Mega Fat CEO Baby's idea when his failed. Which turned Mega Fat CEO Baby into a selfish, spiteful jerk.