As Madison had it, a league was an agreement among sovereigns, while a constitution made a single entity. The former could be dissolved by the sovereigns, while the latter could not.
Note that this was not the explanation of the Constitution given by the Federalists in the Virginia Ratification Convention. They described it as Vattel described federal constitutions in his highly influential treatise on the law of nations. See my account of Nicholas’s speech in the Richmond Convention’s waning hours.
(A compact could be either a league or a federal constitution. The point of using that word is to convey the fact that there is more than one party.)