Hahaha na no real wise words, been said already exaggerate the sizes, a huge helium balloon and a small bag of sweets would lift the bag.
So yes it could "lighten" your bag and its not a stupid comment to make, but in reality the difference would be very marginal.
its explained further here, http://en.wikipedia.org/wiki/Lifting_gas so you could work out how much helium it would take to lift your 300g of sweets but I cant, all day revision means my maths ability is below a 5 year olds, took me half an hour to sort a friends question about logs today haha
Edit to lift 250g of sweets you would need about 40 grams of helium how much volume this would occupy would depend on temperature and pressure.