Deleting PDF page make document larger?

I took some PDFs and started deleting blank pages and was surprised to see the document sizes increase after deletion. Basically doubling in size. A 55K image is now 115K. Then I deleted another page and it went down to 95K. What’s going on here?

You will probably experience the same after deleting pages via the Preview application as both DEVONthink & Preview use Mac OS X’s PDFKit framework.