Kilograms (kg) are 1000 times bigger than the same number of grams, so we need to convert the kg into g.
600g, 606g, 660g, 666g. Now that all measurements are in the same units, then the order is:
600g, 606g, 660g, 666g. We can restore the original measurements:
0.600kg, 606g, 0.66kg, 666g, which is the original list, and it was already in the right order!