Ever tried squeezing too-tall spaghetti into a tiny pot? Perhaps you broke the spaghetti in half to make it fit?
By using a pot that's not large enough, the water temperature drops significantly when the pasta is added. More so than if you were using a larger pot. While the water returns to a boil (which can take a while), the pasta gets clumpy and mushy sitting in the pot. This also creates a higher starch to water ratio, which makes for sticky pasta.
