Do we get what we pay for?

So I bought this Cadbury Hot Chocolate for R47,99 at Spar and when I opened the bottle it was only half full...now I want to know is it false advertising having a 500 or 750g jar that is paid for and when it's opened only half of the jar is filled with contents of the product, or is it a bigger jar and 500 or 750g product inside. Would someone from Cadbury please be so kind to explain this...