Last time I checked, as a result of the checks and balances worked into the constitution, the judicial branch (which the SCOTUS is the head of) and congress (along with the executive branch) were co-equal branches of the government.
Three branches are equal only in the sense that they are the three branches.
Congress has power over the executive and judiciary through legislation (including increasing/decreasing number) and impeachment powers.
SCOTUS can decide on the constitutionality of legislation, but can also be impeached by congress
Neither SCOTUS nor the president can remove members of congress, only congress can.
Thus congress is the supreme power, subordinate ONLY to COTUS.
Hence "a republic ma'am, if you can keep it". The founding fathers recognized the danger of an overly powerful executive and an overly power judiciary