Most of the weight is fuel/propellant, which is why most Mars mission plans have you manufacture propellant on-site. An empty fuel tank and some engines isn't that heavy. Especially if, as you say, you're able to reuse your lander. Anyway, everything you bring has weight. The issue is, how much and can you budget for it?

If your looking for somewhere to save weight, imo start by getting rid of the astronauts and all their associated life support and living space. Bonus - robots don't even need frivolous luxuries like getting to return home.