Huge jump in file size from xara file to PDF export

[WideEyedLife]

I have a (roughly) 8 1/2" x 11" double page spread (ie., two 8.5 x 11" pages side by side, plus bleed margins) that I'm trying to export as PDF. (I've attempted to attach the document to this post.)

Currently, no matter what I try, the files balloon in size upon export to PDF.

Each page contains one JPEG each, with a file size of roughly 1 mb, for a total of 2 mb, pre-PDF export.

I'm exporting as PDF/X, have disabled "optimize for fast web viewing" (this is for a book, going to press); in the objects tab I have kept the default of "native" compression, set at 85% quality, dpi is set at 300; in the layers tab I have disabled "include crop and printer marks in PDF document".

When I export in this manner, the resulting PDF is 17.5 mbs!

The printer I'm working with has a 400mb limit for the entire interior book file; at this rate, I can only have a 20 page book! Does anyone have any idea what I might be doing wrong?