You're forgetting the other "biggest land grab in history:" Russia, and most of the population were serfs - virtual slaves, in other words.
Your theory doesn't hold water.
If you want a clearer example, just consider North Korea and South Korea. How do you explain the difference there? How about East and West Germany?
I explain the difference by pointing to how the United States of America began with an intent to be different from that which we claimed our independence:
"For all (...of Americans'...) differences, they shared that one brave idea (...of the United States as a new world to start again...), and that idea became the point around which they gathered. After two hundred years this is still the glue that keeps the nation together. Its a fragile construct. And the constant American need for reaffirmation of Americas greatness - their exceptionalism - affirms its fragility."
-from A Look At American Exceptionalism by Martin Sellevold, (2003). Australian Rationalist (Croydon, Victoria, Australia: Rationalist Association of Australia, Ltd.) (65): 4648.
& to claim that Capitalism is the sole reason for the US being the largest economy in the world is an extraordinary claim. Extraordinary claims require extraordinary evidence.
I just posted the evidence. Explain the difference between North Korea and South Korea if it isn't capitalism.
Whatever the original intent of the Founding Fathers was is irrelevant. Economics doesn't care about intentions. It only cares about how the social order affects output. Socialism produces poverty. Capitalism produces wealth. The evidence is irrefutable.
I would prefer to speak about the United States of America. We have a mixed market economy as do most advanced industrial Countries. We don't have pure Capitalism or pure Socialism here or elsewhere. It's not irrelevant that what we have here is approaching a modern day Feudal System.