The apportionment of electoral votes is based on the congressional representation for each state, meaning that each congressional seat equals an electoral vote. Since the House of Representatives is set at 435 seats and the Senate currently has 100 members, changes in electoral votes with every 10-year census are often very minute. Therefore, the number of people per electoral vote in one state is very different than the number of people per electoral vote in another.
Below is a list of states along with their populations, number of electoral votes, and a percentage that demonstrates the relative value of a vote cast in that state compared to the national average For example, in 2008, on average a state is awarded one electoral vote for every 565,166 people. However, Wyoming has three electoral votes and only 532,668 citizens (as of 2008 estimates). As a result each of Wyoming's three electoral votes corresponds to 177,556 people. Understood in one way, these people have 3.18 times as much clout in the Electoral College as an average American, or 318% (as listed in the pdf chart, downloadable below).
2008 Population vs. Electors, State-by-State information (pdf, 60.5kb)
This is just one way of looking at the Electoral College and the way voters are represented in presidential elections, however. A better way of measuring a voter's clout in a presidential election is simply whether or not they live in a swing state. Those voters that live in "spectator states" essentially have no clout, as candidates know that the outcomes in those states are already decided. There is very little incentive to campaign in those states or respond to the needs of those voters.
National Popular Vote provides analysis of this spectator status for most voters by highlighting the fact that most of the 2016 presidential campaign took place in just 4 states. Rob Richie and Andrea Levien of FairVote documented this inequality in the 2012 presidential campaign in a Presidential Studies Quarterly article in 2013 as well.