In 1787 the office of the President was created with the creation of the 2nd Constitution. This office, it was felt, was needed to help run the United States and enforce the laws laid out by the Congress. If we look at the Constitution, article II is shorter than article I suggesting the framers intended more power, and thus more restriction, upon the Congress.
The power of the executive comes from those that support him. People grant the executive power when decisions need to be made NOW and for Everyone. When the environment allows for more deliberation power shifts to the large body. So historically we can point to several contrasts. England and the US on one side, France and Russia on the other.
England and the US are effectively islands. The US is continental, but with Mexico and Canada as its neighbors it was never under threat (before the 20th century). England is the lower 3/5th of Great Brittan. The Scots were rarely a threat and after King James I the two kingdoms were politically bound. Opposed to that are France and Russia. France is bordered by Spain, Italy, the Low countries, and the Germans. Russia by the Germans, Swedes, Turks, Mongols, Eastern Romans, and internally very large with many people who sometimes rise up.
Both France and Russia are constantly under attack in history and having to push back and outward to maintain itself. These military demands granted the monarch, the executive, more and more power. England and the US are rarely under attack. Invasions are rare, internal insurrection is uncommon. As a result both places developed stronger Legislative Bodies at the expense of the Executive.
The US president was very weak until the 20th century. What happened in the 20th? Well, it started off with America conquering an empire in 1898, then two world wars which lead directly into a 40 year long Cold war with the possibility of total annihilation. That ended in 1992 but backed right into the War on Terror, the first real invasion of the USA since 1812.
These events saw the creation of a standing army for the first time in us history. Coupled with the Progressive movement and social demands on the state, the combination of Military and Social demands has grown the state and the Executive power to levels that King George III would envy.
In the last 5 years, Executive power has taken much greater leaps and bounds by breaking free from Congressional checks. Laws are simply ignored, rules violated, and no one is holding the Executive’s feet to the fire. While the next president or the one after might follow the rules and checks and balances, the precedent has been set and it has been shown that with the right kind of Charisma, dictatorial power can be assumed.