Exporting a single page or subset of pages produces file of nearly same size as original file. Starting with a pdf of 21 pages with a size of 15205775 bytes I exported a single page. The file size was 15204344 bytes. I exported another single page; it was 15204344 bytes. I exported a subset of three pages; size 15204373 bytes.
Opening one of the pages in Okular and printing it to pdf file resulted in only 183709 bytes. Doing the same for the set of 3 pages resulted in 585068 byte file.
Original: 15205775 Jun 9 13:18 multiPagePdf.pdf
Export 1 page: 15204344 Jun 10 14:19 multiPagePdf01.pdf
Export 1 page: 15204344 Jun 10 14:19 multiPagePdf02.pdf
Export 3 pages: 15204373 Jun 10 14:24 multiPagePdf1-3.pdf
Printed Export 1 page: 183709 Jun 10 14:27 multiPagePdf01.1.pdf
Printed Export 3 pages: 585068 Jun 10 14:29 multiPagePdf1-3.1.pdf