I just tested with a few images and the size depends on the content. For example 36MPx NEF, original file about 40MP. I got xmp files from 16MP to 38MP.
What seems to be a better option is to export as DNG. Instead of 40MP + 38MP for NEF + xmp, I get 61MP for the DNG, which includes de denoise information. These DNGs are undemosaiced, so they are considerably smaller than the Linear DNG generated in the previous version.