Why were the Founders so specific about which powers Congress would and would not have?

The main concern was how to strike the balance between the powers of the federal government and those of the states. Another major concern was that of representation. How the states and their populations were going to be represented in order to prevent neglecting the interests of smaller states? The Great Compromise was reached; a two-chamber Congress with a House of Representatives with members who would be elected according to the population size in states, and the Senate with equal representation for all states.