Perhaps the most important change in the executive branch since its inception has been the huge growth of the federal bureaucracy.
When George Washington was the first president under the new Constitution, the federal bureaucracy consisted of a few hundred people. There were only three cabinet level departments. Nowadays, the federal government employs over 2.5 million people. Of course, the country is much bigger than it was in those days. However, the population now is only about 100 times bigger than it was in those days and the increase in the bureaucracy has been much greater than that.
The major change then, is that there is a much bigger bureaucracy that gives the executive branch much more of a role in creating and executing policies than it originally had.