The concern that led to the Constitutional Convention was that our government under the Articles of Confederation was too weak to defend itself and to maintain a functioning economy. So we created a Presidency with more power, and two other branches to guard against abuse of that power. They addressed concerns about the possibility of a King by including checks and balances. Veto override, impeachment process, judicial review and the power to constitutionally amend protect against abuse by the President.
The Presidency has grown because the country has grown into an empire. The President controls a much more powerful military and has the means to use it without a declaration of war. The laws have been changed over the years to encroach on civil liberties, almost at the President's discretion, especially since the 20th century and 9/11.
From the most fundamental point of view, the office of the executive was seen as something that needed limitation and control. The fear of the Framers, as arising from the Revolutionary War, was the reality of an unchecked executive. This might be the reason why the Executive is limited, as the other two branches are, through the system of checks and balances. This has not been able to undergo much change over time. Where the powers of the Executive might have seen some pliability have been in moments of war and instances where the country was placed in peril from different elements. In the end, though, the framers' vision of an executive branch that is limited by the other two branches has been fairly well maintained.