There's a common misconception that the group which everyone calls the Big 6/5 US publishers are the largest publishers in the world. That's simply not true, and as you can see in the list which PW released last week there are many publishers larger than the US publishers which get the most press.
On Friday Publishers Weekly released their annual list of the world's largest publishers. The list varies only slightly from last year, and it reminds us that trade publishers like Macmillan, PRH, S&S, HarperCollins, and Hachette aren't nearly as big as we might think.
According to PW, the world's 5 largest publishers consist of 4 technical and academic publishers and Random House (aka PRH here in the US). Hachette and Macmillan come in at 6 and 7, but only because they are counted as part of multinational conglomerates (Hachette Livre and Holtzbrink).
Coming in at #16, the next major US trade publisher is HarperCollins. They were beat out by a number of other publishers in the US, including Wiley, Scholastic, HMH, and McGraw-Hill.
And then there is Simon & Schuster, which ranked as the 28th largest publisher in the world in 2013. With $809 million in revenue, S&S is a fairly large publisher in the US market but they don't have much in the way of operations outside the US. In absolute terms they are small fry, which is why I am expecting one of their larger competitors to gobble them up in the next couple years.
You can find the complete list over at PW. I would suggest bookmarking it so you can come back later (I plan to do so).