One way to build something big is to build something big. Like a pyramid. Or a multifaceted bank. Or a university. Another way is to build something that is architected to be full of small pieces, small things, small units.
Khan Academy is one of these "build small things" that became big services. Another is Quizlet. These services, among others, have probably done more to advance knowledge for millions of people than anything else in the last decade. But I think that they work not simply because they advance learning or that their respective missions are to offer education to as many people, anywhere, and for free, as possible.
In fact, they may work in spite of those lofty goals. Instead, they work because they start small in what they can accomplish for an individual user. Quadratic equations. French animal names. Telling time in Spanish.
At a specific level, they each work in a way that is consistent with how people think and, 20 years into the web, desire to find information. For example, someone may think to herself, "I forget how to subtract fractions." They then conduct a search for it, and Khan delivers a 4 minute video lesson. The whole process may take 5 minutes and is hardly interruptive.
Similarly, on Quizlet, a student will think, and search for, "-AR verb combinations in Spanish." Flash cards appear and the lesson is on its way.
Then, in aggregating all these small pieces, something much grander and transformative appears. Before anyone realizes it. Knowledge and sharing bases that are used by millions and millions of people.
(Khan, of course, has historically centralized production of this content. Quizlet, on the other hand, has a peer produced model.)
This morning I was reading an incredible article about a company that reads Amazon reviews to find out what consumer electronic products people want - and then manufacturers and sells them in small batches. This company's customer research, if you will, is to simply listen to what people are saying on a micro level on another service. I want a bluetooth speaker for my shower. It thereby can and does compete against the behemoths of its industry. "A tiny company with one speaker can compete against anyone."
Small things can mask grander ambitions. They also can be an amazing way to create something really big.