I extracted one page of a document with 106 pages and 13210KB of size. The new document, composed by the sole page I extracted, has a size of 13615KB. How is that possible?
I also tried to do the reverse operation, i.e., removed all pages of the document except the one I wanted to keep and the result is the same.
That occured in both offline app and online.
I suspect that the PDF was built so that each page references and includes all of the data in the PDF, and thus the PDF does not get smaller. But I can only say more exactly when I have seen the PDF. It may help to reduce the size of the PDF beforehand to resolve the references.