
How fattening the milk is, relates to the amount of fat (not just
sugar) it contains. While sugar plays a part in adding more
calories, sugar is a carbo which is about 4.3 calories per gram
(?), while fat contains about 9 calories per gram.
So, weight by weight, 100g of butter x 9 cal = 900 cal, while 100g
of sugar contains about 430 cal. That's why ppl say butter is more
fattening than sugar.