Hi Kovid,
Thank you for your quick answer, you are right for most cases, but in my particular situation, I think this is not completely true: I am working with history books that contain hundred of references (other books, citations, etc.).
I did a short test, and not with the book that have the most footnotes:
- docx converted to epub: 1318 KB, 1298 sections, 7 s to load in calibre viewer,
- the same epub, with notes sections merged: 683 KB, 56 sections, 2 s.
I do not think I am not saving anything by reducing the page count
Since I cannot do that directly and this is very particular, I will merge sections manually after conversion.