There is a debate going on downblog over the United States Senate.
Let's take a brief tour of Senates around the hemisphere. (To which we'll include the Philippines, due to its political history.) The American example set in 1787 proved very influential, particularly for the larger nations of América. Did their adoption of an upper house with equal representation for federal subunits make sense? Did they keep it?
Argentina: The Argentine senate gives each province three senators. That's obviously undemocratic. Argentina, however, has a historical justification for the practice. Like the United States, the Argentine Republic began as a federation of independent states. Unlike the United States, the states that formed the Argentine Republic had very little autonomous political history; most had been created from city councils that simply declared independence in the wake of the political chaos that enveloped South America after 1808. After a great deal of civil war, including the loss of Bolivia, Paraguay, and Uruguay, provincial forces decisively defeated Buenos Aires in 1852. A convention that excluded representatives of Buenos Aires hammered out a constitution the next year. That same constitution has remained in effect since then, at least juridically.
The convention accepted equal provincial representation in the Senate in order to limit Buenos Aires's dominance. It also had senators elected for nine-year terms by state legislatures and required them to be able to show a minimum annual income of 2,000 pesos. (That's approximately US$67,000 today in terms of purchasing power.) Thankfully, later amendments got rid of those latter features, but equal representation remains. In short, while there isn't really any good reason to keep the apportionment of the Argentine senate, the institution has deep roots in Argentine political history. The resemblance to the U.S. case is quite strong.
Brazil: The senate of the Federal Republic of Brazil gives three senators to each state. The historical roots of this institution, however, are rather shallower than in Argentina or the United States. The 19th-century Imperial senate did not provide equal representation for the provinces. When the empire fell to a military coup in 1889, the new leaders had to find ways to convince provincial elites to support the new government. One of the ways that they did so was to provide the provincial governments with equal representation in the upper house, in open imitation of the American example. (In fact, the constitution of 1891 named the country the United States of Brazil.) Brazil has gone through multiple constitutions since then, but the institution of the Senate has remained. Unfortunately.
Mexico: The constitutions of 1824, 1857, and 1917 all granted equal senate representation to each state. There was never much historical justification for doing so, other than imitation of the United States. (Mexican states gained much autonomy during the 19th century, and several declared independence, but that came about after the end of Spanish rule.) Since the country's governments rarely ruled by the constitution, however, legal stipulations didn't matter. Under Porfirio Díaz, for example, all legislative votes were unanimous after 1893. Nor did senators necessarily come from their putative state. Under those circumstances, the number of senators per state didn't make a difference. The Revolution didn't change that. Under President Álvaro Obregón, for example, recalcitrant legislators were often shot. Then came the creation of the PRI, with its iron-lock over politics, and ‘nuff said. Constitutional stipulations didn't become more important than party rules until the 1997 midterm election.
Mexico, however, is now ruled by a democratic government, and the constitutional rules matter a lot. As part of the transition to democracy, Mexican politicians amended the constitution in 1993 and 1996. In theory, the reforms granted each state four senators, up from two. In practice, the reforms created an upper house selected by a weak form of proportional representation. The winning party in each state receives two seats. The runner-up receives one. The remaining 25% of senate seats are then allocated to each party in proportion to their popular vote share. In effect, the Mexico's current senate keeps the form of equal representation for each state while abandoning much (but not quite all) of the substance.
The Philippines: The Philippine Autonomy Act of 1916 created a 24-person senate. The act grouped the country's 50 provinces into 12 districts, each of which received two senators. The 1935 constitution of the Philippine Commonwealth dumped that crazy system, but selected something even crazier in its place: at-large Senate elections. That is, every voter writes in 12 names. The top 12 vote-getters then become senators for six years. The exercise is repeated every three years, making for a total of 24 senators. The 1987 constitution of the Philippine Republic retains the feature.
The only polity that I can imagine justifiably imitating the United States, Argentine Republic, and Federal Republic of Brazil in the 21st century is the European Union. Then again, I may suffer from a lack of imagination.