Factors in the growth of presidential power?

1. Congressional deference: It is easier to let another person do the job (i.e. the national budget). Congress was originally supposed to have set the budget for the nation, but decided to hand that job over to the new president in office. In this way they themselves have expanded the Power of the president.
2. Willingness of a president to set new precedents. For example, Andrew Jackson began using the veto in a new way. Earlier presidents used to veto only when the believed a bill violated the Constitution. Jackson vetoes legislation just because he thought it was a bad idea. Presidents now order military strikes that early president swould not have considered doing without the support of Congress. Obama has decided not to enforce parts of the Immigration law, thereby in effect changing the law.
3. Increased power by the federal government. The federal government now is now deeply involved in areas such as education and social services that used to be left to the states and the feds can easily override state laws. The president gains power whenever the federal government gains power.
4. Improved communication media: The President can go on TV and present his case to the nation. If he a persuasive speaker, he can people to appeal Congress to do what the President wants.

Note that these reasons are all related and intertwined. It is not always clear which are the cause and which are the effect.