In general , a large , slim-bodied macropod of the open plains is called a `` kangaroo '' ; a small to medium-sized one , particularly if it is relatively thick-set , is a `` wallaby '' : most wallaroos are only a little smaller than a kangaroo , fairly thickset , and are found in open country .
Most wallaroos are smaller than a kangaroo , and live in open , rocky country .