It's not just the pure numbers, but geographical and cultural range. 8 million people in an area (16k sqmi) smaller than the US State of West Virginia(24k sqmi) are going to have an easier time understanding and voting on issues affecting everyone than 300+ million people in an area over 9 millon square miles. Direct democracy in the US could end up with even worse tyranny of the majority issues than already experienced. (Example, detonating multiple nuclear weapons in the state of Nevada in the 50s. It might have passed in a national vote, but likely not a vote only in Nevada.)