Why not give them votes according to how much money the country is delivering to the common budget. It could be gross money or net money. If it is net money then the spending will decrease faster.
Because the EU should not have budget at all. Anyway this would be the way to complete marginalizing of the new member states. If such argument would be clearly stated before they joined, perhaps they would stay outside. People who voted in all these referendums would be cheated.