Adam Ziegfeld in The Washington Post's The Monkey Cage:
First, as of the most recent counting, almost 70 percent of Indians did not vote for the BJP. Commentators such as Max Fisher at Vox claim that the BJP “dramatically … swept the vote.” In fact, the BJP won about 31 percent of the vote, a new high for the party. Although this is the first national election in which the BJP has ever won more votes than any other party, less than a third of Indians voted for it. The BJP’s legislative majority is largely a function of India’s single-member district (SMD) electoral system, the same system used in American, British, and Canadian legislative elections. In an SMD system, votes rarely translate proportionally into seats. This system rewards parties that are the largest in each electoral district. The BJP’s vote is patchily distributed across India, which works to its advantage. In a number of states where it is disproportionately strong, the BJP was, in district after district, the largest party, even if not always by a very large margin. For example, in India’s largest state, Uttar Pradesh, the BJP won about 42 percent of the vote. However, it is likely to win 71 of Uttar Pradesh’s 80 seats (almost 90 percent) because the remaining 58 percent of the vote was split across a number of different parties.
Meanwhile, in states where the BJP won few seats, it did quite poorly. Thus, relatively few of the BJP’s votes were wasted—that is, cast in electoral districts where the party ultimately failed to win a legislative seat. As a result, the party won a legislative majority on a fairly small vote share. Previously, no party had won a legislative majority with less than 40 percent of the vote. Meanwhile, Congress suffered an ignominious defeat in part because its vote was fairly evenly distributed across the country. Coming in second or third place across many electoral districts brings no electoral reward. In this election, Congress looks a lot like the Liberal Democrats in Britain—a party that typically wins respectable vote shares in lots of districts but fails to win many seats.
More here.