One of the major factors underlying American economic growth in the early 1800s was the expansion westward, possible thanks to the Louisiana Purchase, which gave American farmers access to vast tracts of arable land and a reliable transportation link to the port of New Orleans in the form of the Mississippi river. The access to a viable trading port encouraged production and production in vast quantities, the kind that couldn't be used locally but thanks to shipping south and then out of New Orleans, farmers could manage. So farms grew much larger from the smaller homestead-type farms to the plantations of the south and the larger managed farms of the northern states.
In my opinion, the major factor (above all others by a long way) is the expansion of the transportation network in the United States.
During the early 1800s, governments in the US started to create a larger transportation infrastructure. Roads were built and, more importantly, canals were created. The canals, especially, allowed products to be moved from place to place much more efficiently. As the price of transport dropped, the size of the American market grew.
The size of markets is very important for economic growth -- this is one reason countries want to make free trade agreements today. When transportation became available, producers could, for the first time, have access to a market that stretched far beyond their local area. Producers in the inland US could sell to a much larger market than ever before.