The Electoral College allocates electors to states based on representation in Congress, so a vote for president in a less-populated state is “weighted more” than a vote in a more-populated state; the less populated a state is, the less individual votes are needed to swing a single electoral vote. This has always been the case, but the gap today is larger than it was in the early history of the country.
In the 1796 election (when Washington left office), a vote from the least-populated state (Delaware) was weighted 1.75 times that in the most-populated state (Virginia). In other words, the votes of a Delawarean was nearly powerful as two Virginians.
In the 2016 election, a vote from the least-populated state (Wyoming) was weighted 3.6 times that in the most-populated state (California). A vote in the smallest state compared to the largest has roughly doubled in power since 1796, where a single Wyomingite’s vote is nearly four times as powerful as the vote of a Texan.
This widening gap can largely be attributed to the Apportionment Act of 1911, which capped the House of Representatives at 435 members. Constitutionally, the number of presidential Electors must match the number of representatives in the House.
If the House had continued growing in accordance with population, then we would have several hundred more representatives. With larger-populated states – California, Texas, New York, Florida, etc – having more representatives (and therefore more electors), the vote-weight gap would look more like it did in 1796.
Picture is weights for the 2016 election. My population numbers are based on the 1790 & 2010 census data.