The Quantity field is used alongside the item prices feature to calculate the total cost of an item, so it is intended for entering the number of packages or the amount by weight to be purchased.
Information about the package size (e.g. 12-oz can, 2 liter bottle) or an amount needed for a recipe (1 cup, 2 Tbsp) is best entered in the Notes field.
When purchasing an item sold by the kilogram, if you need an amount less than 1 kilogram (e.g. 400 grams), you would enter the amount in the Quantity field as 0.400 kg.