Under the Articles of Confederation, all states were represented equally in the legislature, under a "one state, one vote" concept. Thus, smaller states had just as much effect on legislation as larger states with more population. The founders sought to fix this issue during the Constitutional Convention in 1787, by allocating each state representatives in Congress proportionally, according to population. This meant that larger states would have more representatives, and thus, more influence over legislation.
Obviously, the smaller states did not care for this proposal, and fought it during the Constitutional Convention. Delaware vehemently objected to the elimination of the "one state, one vote" principle. They believed that proportional representation in Congress would give the larger states too much power, and they also feared the influence of the rapidly growing Southern states. The delegates from Delaware were given very strong instructions in this regard,
"They foresaw the annexation of small, ineffective states as the populations of the large states continued to grow and their influence waned. Some, like the Delaware delegation, were instructed to leave the Convention if equal suffrage in the legislature was compromised." (www.usconstitution.net)
Motivated by the fear of the growth of the Southern states voiced by Delaware and other smaller states, the Connecticut Plan proposed counting slaves as 3/5ths of a person for purposes of representation in Congress.