Unmanned missions can be far lighter than manned missions. Once you add food, water, air, and a pressurized habitat, you’re upping the bare minimum mass of the mission by a hundred fold, or a thousand fold if you get ambitious about it.
The Viking missions had a total mass of only 2-3 tonnes. The lightest weight manned Mars mission study called for a mission mass of 118 tons, and got away with that only by using a nuclear rocket and not putting the main rocket into Mars orbit. A full up Mars mission (entering Mars orbit) called for a total mass of 2100 tons.
