> It's the Supreme Court that has expanded the powers of the President

Sort of, but Congress also wrote a bunch of pretty broad, vague laws, delegating a significant amount of power to the executive via agency rulemaking, and it turns out the agencies are part of the executive branch and have to do what the head of the executive branch says they have to do (within the limits of those broad, vague laws). If Congress can't get back to smaller, simpler, more specific laws, and they continue to pass the burden of this complexity over to the executive branch to figure out, the executive branch will continue to wield outsize power.