<<Where is this "rule" stated?>>
I don't know where it's stated. During the Civil Rights era, it was "one man one vote" so I guess the version I just gave is the more PC one. It's a principle by which a lot of people evaluate how democratic an institution really is.
<<For that matter, why is Congress allowed to stand? After all, the delegation to Congress is just as "unbalanced" as the Electoral College...>>
Well, I guess Congress is more obviously a relic of the Federal system. Seems to work pretty well as it is, so "If it ain't broke, don't fix it." The Electoral College is also a relic, but obviously a broken one if we look to the 2000 Presidential election, which was a catastrophe. The people didn't get what they wanted, they got what the System wanted, and the result was a disaster, compounded by the fact that most people didn't vote for the idiot who represented the minority of voters but managed, due to the System, to foul the nest that they all lived in.