How can deleting pages from a PDF result in an increase in file size?

Question

I routinely scan pages to PDFs and then use Abbey FineReader to run OCR on those PDFs, creating a new PDF which contains both the original images (further compressed) as well as the recognized text in something like an invisible, searchable overlay.

There's an extremely strange phenomena where if I delete some pages using Preview and save the file, the file size increases instead of decreases. E.g. a 250KB file just turned into a 650KB when I deleted one of its pages...

Any idea why this happens and how to delete pages without such an increase?