"I'm not debating that largescale changes in economy can happen in theoretically democratic countries, I'm just positing that the former USSR's mechanisms took less people to enact them. Stalin did a lot of things with the economy that were short-sighted and punitive, like creating a famine in the Ukraine. While a presidential candidate with widespread support can bring a lot of like-minded people to Congress to assist him, it still requires more people to go along with it than the Politboro did. In just about every vote that's at all contested, a few people vote against their party line. And there's been cases of legislation pushed forward by one party that the sitting President, of the same party, has vetoed."
Except the NRA was put into action by executive order (later ruled unconstitutional). And sorry I have to disagree with your characterization of Soviet decision-making. Also while you may have a general point about the American presidency, parliamentary governments are not burdened by internal divisions within the government. If the Prime Minister proposes a policy, it is law. If there is sufficient opposition to block it, new elections are held. In the UK these days, the Prime Minister doesn't even have to do as much as consult his cabinet.
And even in the case of the US presidency, some pretty draconian policies were implemented by executive order (in total Roosevelt issued 3,723 EOs) - the most notorious of which was EO9066, pertaining to the internment of people of Japanese descent. I also don't think anyone can really say that decisions on issues pertaining to foreign and military policy are constitutionally a collective matter when it comes to the United States. They are, in fact, collective matters when it comes to the Soviet Union. The best example of which is the intense debates that took place before the USSR finally committed troops to the defense of Afghanistan.
In the USSR few decisions were ever made by one individual. No single Soviet leader could "veto" a law passed by the legislature. No Soviet leader had their personal authority written into any constitution or law, like the dictators of Italy and Germany. On the contrary, the Soviet government explicitly frowned upon individuals making decisions by themselves. Decision-making in the USSR lacked the decisiveness and swiftness of an individual dictatorship.
And when it comes to that favorite bugbear Stalin, several things must be pointed out. Stalin was not the President of the Presidium nor even a cabinet official in the government, nor was he president of the Central Committee of the CPSU. He was General Secretary. The idea that he could singlehandedly direct all decisions made in the vast Soviet governing apparatus from that position is far-fetched.
Furthermore, even his critics admit that Stalin never sought attention and recognition for himself.
"Stalin does not seek honours. He loathes pomp. He is averse to public displays. He could have all the nominal regalia in the chest of a great state. But he prefers the background. He is the perfect inheritor of the individual Lenin paternalism. No other associate of Lenin was endowed with that characteristic. He lacks culture, but he absorbs knowledge. He is rough toward, his enemies, but he learns from them." (Isaac Don Levine, 1929).
He didn't write long winded fantastic tales embellishing on his supposed role in the October Revolution like Trotsky. He was content being a figure in the background. That's how the entire Soviet leadership was, it was a collective body, not a group of individuals. The General Secretary essentially existed to carry out the orders of the Central Committee. The person holding that position worked at the mercy of the Central Committee and could be dismissed by it at any time - this is precisely what happened to Khruschov in 1964.
You bring up the famine (which I've discussed ad infinitum here, but oh well) - the famine was not the result of a 'punitive' action. The famine was not local to Ukraine, where kulak resistance was greatest. The famine was widespread, and affected Kazakhstan, the Caucasus, West Siberia, among other places. Places which did not have significant resistance to collectivization experienced famine, because there was widespread crop failure. Little known fact, but the prosecutor of the USSR - Andrei Vyshinski, had hundreds of thousands of kulaks released from prison and their voting rights restored, and the NKVD was thereby prohibited from arresting anyone without consent from the prosecutor. A Ukrainian historian, considered the 'leading Ukrainian historian' by Ukrainian nationalists - Mikhail Hrushevsky, does not make any mention whatsoever of a deliberate man-made famine (punitive action). Instead he rightly says that "A year of drought coincided with chaotic agricultural conditions, and during the winter of 1932-1933 a great famine, like that of 1921-1922, swept across Soviet Ukraine". (Hrushevsky, 1986). Yes excesses occurred against kulaks, but this was not pre-meditated policy of starving Ukrainians.