bearing in mind that 1Kg ≈ 2.2lbs.
so, if a baby is say 2.2lbs, and then it doubles in weight, to 4.4lbs.
well, we know when the baby was 2.2lbs it was exactly 1Kg, but when it went up to 4.4lbs, is it 2Kgs? well, yes, 2Kgs = 2.2 + 2.2 or 2(2.2).
so, multiplying the lbs value by some factor, will retain the Kg:lbs ratio.