Technically, the diameter ratio with pulleys works same as in sprockets. In sprockets they use teeth ratio terminology under assumption that teeth pitch is standard and number of teeth is proportional to diameter. So teeth math is an indirect way of doing diameter math. In engineering, base for all RPM ratio calculations, belts or chains, is pulley/sprocket diameter ratio. Not teeth nor ribs. Diameter is what counts.
Belt pulleys will always look bigger because they have a raised channel to contain the belts and prevent them from coming off. This is not to say that this pacticular counter pulley in this picture was not made bigger than stock sprocket to lower RPMs.
