Depending on the fabric, dresses can be altered and made bigger but not usually by more than a size or two. It is always easier to make a dress smaller than it is to make it bigger.
One of the most common ways of making a dress bigger is to let out the seams. The seam allowance must be wide enough to allow for re-sewing of the dress with a narrower seam allowance, and the fabric must be resistant to pin-marking otherwise the previous stitching lines are visible. Another option involves using fabric inserts to add size to the dress.