Ah, I didn't realize they were sleeping PJs. That would be a problem, as our dog can slip out of a harness unless it is skin tight.
Perhaps you might think about pinning, marking and making alterations to this garment, with basting stitches, and then taking it apart in its altered form, and using that as a pattern. Idea #2, cut out the pattern between the two sizes. Who would have thought sewing for the dog would be like sewing for ourselves, but they all have different frames, etc.