Preview: reorder pages in pdf increased file size?
I found out that it has to do, in my case, with embedding Types. 
after combinig them with preview or (I use a lot of chapters) the free tool PDF Merger Free (or in your case, reordering), it seems they swallow the whole fontbiblio (well, guess not, only feels like), so I drop them on the app PdfCompress (https://www.macupdate.com/app/mac/8823/pdfcompress), in which preferences I choosed, without touching the compression Size (just in the middle), as fonts base 14, or none (this is risky, don't do it without checking your whole Dok afterward, makes funny things if the fonts aren't standard).
Hope this helps...