i have to disagree with population = delegates. UN doesn't work that way, NATO doesn't that way etc. in that case, world congress would improve position of strong players, but i think it'll be more like colective rule with everyone on the map (or with an original capital) with the same vote as other civs. (with the fact, that powerful player will have easier time getting away with sanctions.) also as you say, this will be another blow for tall civs, so i don't think that this will be the case.
The UN and NATO aren't the right comparisons. Compare the World Congress more to the Congress of Vienna, where everyone and his dog in Europe attended, but all the decisions were made by Austria, Great Britain, Prussia and Russia, with a bit of input from France.