A small robot could hypothetically travel as far as it's fuel allowed, but it would need large tracks (or wheels) in order to navigate any obstacles or go on any worse terrain then a paved highway as well as, as you point out, a larger body to store the fuel, but also to house a larger engine because a long range means nothing if it can only crawl by at 7 km/h.
Another problem holding back development of long range UGV's is reliability, what if it breaks down when it's 100 km away? You just cant send someone out to repair it in half an hour. Also, as with Iran's UAV's, there's the problems of communication with the unit, which has been addressed elsewhere on this forum (in the other UAV threads for anyone who hasn't already seen it). This is why i think they'll remain limited to the tactical realm at least for the short-term future.
One day i hope to see roaming gangs of tank-killer drones hunting down unmanned tanks with swarms of UAV's flying overhead. Although, one might worry this might make those with the drones all the more inclined to wage wars on those that don't possess this capability - "We shan't have to leave our fortress now when we want to blow up civilization" - Hank Morgan in Mark Twains "A Connecticut Yankee in King Arthurs Court"