The European Parliament assigns seats to member states based on population, but the European Council gives one vote per state. So the answer is kind of -- they

The European Parliament assigns seats to member states based on population, but the European Council gives one vote per state. So the answer is kind of -- they basically did join, in spite of odd representation mechanics. And FWIW I think the Council (and its majority voting rules) is as much in need for reform in this respect as the US Senate.