Slight expansion in a work/book

""This work is largely based on the late Professor Emile Legouis's A Short History of English Literature, which was first published in 1934 and which established itself as a standard work. There has, however, been some slight expansion: The final chapter of the present book attempts to carry the history of English literature up to the beginning of the Second World War; and an appendix of important dates has been added."

I cannot get the meaning of "expansion" used above. Would you please explain? Which verbs can be used instead of it, retaning the meaning and the idea "expansion" conveys.

Source: An Introduction to English Literature.

Thank you.