I always admire the file size by Xara export.
Yesterday I exported Poster as PDF/X-1 Coated Fogra 39. At the exit I got 80Mb.
Using Acrobat Pro preflight utilité I converted this document in the same format with the same color space. And gotten only 16Mb. Funny actually!
So if you have a way to use Adobe Acrobat Pro, then do the same thing.