The Ganges river basin is fertile--but the most fertile is the basin between the Yangtze and Hu Rivers in China. Though less fertile, the Mississippi River Basin is also fairly fertile. The thing is, is that the Chinese and American basins are huge, India's is not. The most arable region is the northern European plain--but Europe isn't a country. Furthermore, India's Deccan takes up a large part of the country. So to does the Mojave Desert and the Rocky Mountains in the USA you might add. And in that case, it could again be brought up that the United States and China (which has the Gobi Desert and the Himalayas) are larger than India, and that their fertile regions are capable of producing more food.
Also, though your map doesn't specifically state so, it seem to be measuring the percentage of the arable land. 40% of India (1.3 million sq. km) (if the map is accurate--40% is very, very high) is not 19% of the USA (1.8 million square km), or 14% of China (1.3 million sq. km). This is if the amount of arable Indian land is 40% (the range is 40+%), and if China is 14% (although both India and China are about 1.3 million sq. km, China's amount is higher). The United States has more arable land even if it is only 15%.*
See that the map is from Wikipedia, ostensibly from the CIA factbook.
This seems to be another case of Indians not accepting that their country isn't number one at everything. Yes, there are many Americans who have the same view about the United States, including many freepers, but many Americans, and many freepers, recognize that the United States is not perfect (simply number 1 out of many imperfect countries).
India has a lot going for it. Land area isn't one of them.
*Country land area sizes taken from Wikipedia. For India and China, the land area of the territory that they administer was used, not the territories that they believe are theirs (which would largely be Himalayan--see practically nonarable--land).