Why do Picture Frames make for a much larger PDF size than Images?

[GarryP]

Windows 10 Home 1809, Publisher 1.7.0.257.

I have a simple 8-page A4 facing-spread document.
I have 4 small images - total size on disk 136kB - to add as page backgrounds (the same image on the first and last page, different images across the other spreads).

When I add the images as Image layers to the document and export as PDF I get a PDF that is 98.8kB in size. That looks fine and dandy.
However, when I add the images as Picture Frame layers, and export with exactly the same options, I get a PDF that is 2.1MB in size. That's way bigger.

Does anyone know what's happening and why? Am I doing something wrong?
I can attach the documents and/or PDFs if necessary.

P.S. Both of the afpub files are 1.85MB in size, which also seems a little large, considering, but I guess that's a different issue.