While normally for symmetry the eternity ring is kept the same size as the wedding band. It is possible to keep the style the same and just increase the diamond size. If you are going to do this it is a good idea to increase the diamond size by more than a few points, so that the variation looks intentional.
Another option for having a larger eternity ring, is if you wish to wear your eternity ring on the ring finger of the right hand. In this case the ring is usually at least twice the width of the wedding band.