Price per unit, so packs of different sizes can be compared.
Divide each package's price by its weight or volume and compare the resulting unit prices. A 450-gram pack at 2.79 euro is 6.20 per kilogram, while 800 grams at 4.59 is 5.74 per kilogram — so the larger one wins.
No. During a promotion the small pack is often cheaper per unit than the large one, and in some categories the big size costs more even without a promotion. That is why the price per kilogram is worth checking every time.
In Latvia it must appear on the shelf label in small print beside the price. The catch is that one product shows it per kilogram, another per hundred grams and a third per item, and that inconsistency is what makes the label useless.
Convert both to the same unit first: grams to kilograms, millilitres to litres. At 1.29 euro for 330 millilitres you are paying 3.91 a litre, against 3.49 for a 1.5-litre bottle, which is 2.33 a litre.