In a rational society, this practice should be disallowed while keeping the same product name and price.

Either adjust the price accordingly, or be forced to put a big red sign with something like "Now with 10% less content (compared to before DD/MM/YY)".

Even better, products should have their key content-size metric on the box, very visible, normalized by the price. E.g. XX/ounce.

If the retailer makes a discount or changes the price, they should add a label with their changed price YY/ounce.