I like Wiki Answers’s take on the source of Congress’s power to set the size of the Supreme Court:
1. From How does Congress determine the size of the US Supreme Court: “The Constitution does not specify a particular number of justices; Article III gives the power to Congress.”
2. From What was unconstitutional about President Roosevelt’s court-packing plan: “Roosevelt overreached his constitutional authority by attempting to advance his agenda by using authority assigned exclusively to Congress under Article I of the Constitution (determining the size and structure of the Supreme Court).”
This is weak evidence that nobody knows why Congress can set the size of the Supreme Court, and stronger evidence that those least qualified to comment on the question (that is: the Internet) do not know.
It is ludicrously feeble evidence for the view that we are all in the grips of inchoate ideological distortions that we forge into a coherent narrative by half-making shit up. Also known as confirmation bias. From which this paragraph suffers.