Every state gets a minimum of three electoral college votes. Those votes are based on the amount of senators and representatives that state has, Two senators plus a minimum of one representative. Ostensibly this should make the electoral college a simple middleman for the popular vote. Two things prevent this:

Nearly every state (48/50) does winner takes all for their votes, so even if the state goes 51/49 the winner gets all the electotal votes for that state, which in places like California and Texas is a huge swing.
The house was limited in size in 1909, which in turn has given the senate more weight in the electoral college division. 20% of electoral votes are due to the senate, giving smaller states far more electoral weight then they should have.

If the house was properly set by population (lets pretend 1 rep per 100k population.) then somewhere like California would have approx. 400 house members (~40 million pop.), 2 senators, and 402 electoral college votes. Wyoming, with ~600,000 population, would have 6 house members, 2 senators, and 8 electoral votes. Currently those two are at 54 and 3 electoral votes respectively. A ratio of 50:1 in that projection vs. 18:1in reality.

Compound the aforemention representation issues by a bunch of other states that also get that unfair representation, and we end out with a system where the election gets decided by a couple purple states even if the Dem candidate wins the popular vote.