Replying to
E1: You'll need a greater volume of feathers to make a kilogram, but the weight is the same: the weight of 1 kg.
E2: A kilogram is a kilogram, but gold is measured in troy pounds, which are smaller than the usual avoirdupois pounds (0.3732417216 kg vs 0.45359237 kg).
E3: A troy pound is so small because it's only 12 ounces, not 16. A troy ounce is still different from an avoirdupois ounce, but now it's slightly larger (0.0311034768 kg vs 0.02834952312 kg).
(3/3, unless I redo all of this in grains)