DIFFERENCE IN OXFORD AND OTHERS
as someone who attended oxford there are a few reasons for this though it doesnt completely explain 4000 pages.
1 oxford have a huge amount of quizzes and therefore questions and so alot of additional pages in each manual.
2 oxford have a lot of cap booklets built into their manuals in relevant subjects
3 oxford have an extra manual that is called reference material which in itself is quite a large manual.
4 there are quite a lot of graphics in the oxford manual which take up a lot of extra pages.
5 most of the other manual suppliers have 2 manuals for the subject of aircraft general , oxford have 3 manuals.
6 oxford go into far more detail in their manuals than most of the others i have seen.
hope this all helps