I suspect there's some important difference between what we do in our reader CSS and what Firefox does in theirs:

https://searchfox.org/mozilla-central/source/toolkit/themes/shared/aboutReader.css - theirs

https://github.com/tridactyl/tridactyl/blob/master/src/static/css/reader.css - ours

but I can't see anything obvious immediately in the "print" directives.

Is there any way you can tell what is making the files bigger? Is it just images or is there something else?

Definitively the big factor is image size. I checked the images inside a number of PDFs with pdfimages but my observations are inconsistent, sometimes the difference is really small. Other times PDFs generated from a website in Firefox reader mode have images shrunk down to 72 ppi (pixels per inch) while the website, viewed from the corresponding :reader command produce a PDF with bigger images. I don't understand the logic of how it works.

On a second thought, since the PDF printing is handled by the PDF.js library, I am unsure if there is wiggle room for Tridactyl to tune the file size.

When I'll have more time, I can try to investigate further...

On a second thought, since the PDF printing is handled by the PDF.js library, I am unsure if there is wiggle room for Tridactyl to tune the file size.

PDF.js gets some say but our CSS and readability.js also control it too