This is because of proportional representation.
The legislature is composed of the Senate and the House of Representatives. The Senate is considered the state's house, and elects the same number of representatives for each state, so as to maintain the equality of states in their demands.
However, the house of representatives elects deputies in a proportional way, that is, the states with the highest population density elect more representatives than the less populous states. It is a reasonable measure that maintains the proportionality of representation.