They could have been carrying just about anything for ballast, as ships did for centuries. A big grain carrier would ride pretty high on a return voyage.
Casson writes that the return trip of the grain ships, actually their trip both ways, was the preferred way for those in the know (and with the money) to travel to and from Egypt -- the ships were huge, and voyages take time, basically they were the ocean liners of their time, people took sun on the decks, played games, etc.