Someone else can provide the math, but if due to the tanks being mostly empty you can burn anything from practically 0 to 5 G thrust, intuitively you burn the l

Someone else can provide the math, but if due to the tanks being mostly empty you can burn anything from practically 0 to 5 G thrust, intuitively you burn the least total fuel by flooring it to 5G at the last minute. Consider the alternative of burning less than 1G of acceleration, you'll fail to slow down although you'll descent acceleration will be somewhat lower. At exactly 1 G it'll keep falling at a constant speed till it runs out of fuel, then impact. Or if you're asking why performance is so much more spritely at the end of mission, unlike boats or cars the fuel in the tanks is like 95% of the mass of the rocket, so something that barely waddles off the launch pad with full tanks weighs practically nothing at landing when the tanks are nearly empty.