It should make your file size smaller.

When you publish your site, Xara automatically reduces the resolution of all bitmap images to 96dpi, even if the original image is 600dpi. But Xara keeps the images in the file at the original dpi and this can over time lead to a very large document. If you optimize all JPEG images it creates a smaller document file size wise.

Let me say this however. It makes sense to keep the original images at their original file size in another folder just in case you need to scale the images or want to print them.