My total pack weight is roughly 4 kg (8.8 lb) without the radio and without any water. The pack weights includes the pack itself, raincoat (1 lb), a chair (1 lb), antenna mast .75 lb, antenna bag (which includes feedline, wire, rope, transformer, winder, straps) and some safety gear (headlamp, bug spray, etc.). In the grand scheme of things, water is the biggest payload at (1 kg/liter) 2.2 lb / liter and can be a substantial weight in the pack. The KX2/battery/key (720g) versus MTR3B/battery/key (446g), the difference in weight is about 180g or roughly 1/2 lb. The tradeoff is in the versatility of the KX2 - runs SSB (even without a mic), has an ATU, runs 10w and will run all the amateur bands from 80m-10m with an EFHW whilst the MTR3B can only run 40-30-20, needs a resonant antenna and only CW and limited to 5W. On foot - always the KX2, since I can always carry more hiking. By bike, every ounce makes a difference and so I dispense with everything except for the radio and the antenna and everything radio related weighs-in at 2.5 lbs. You can weigh everything in your pack and rationalize:
Do I risk?
getting wet in a thunderstorm versus staying dry? - save 1 lb
risk a breakdown with my morse key? - backup key weighs 54g
being comfortable in light weight camp chair - versus sitting on the ground and cramping? - save 1 lb
do I risk being hungry and thirsty by not carrying enough food or water? - save ~3 lbs (1.5L + food)
One can save weight by taking some risks and certain risks can be prudent - i.e. save a 1lb and not take a rain jacket when the weather forecast has 0% chance of rain. I have used KX3/KX2/K1/MTR for activations, and I have been on foot 98% of the time and 98% of the time I have used a KX2.