How would American sovereignty be lost if the United States became larger? As for free trade, there is already free trade between the states of the Union. Making the country larger would simply increase the number of states. Was American sovereignty sold out when the Louisiana Purchase was bought, or after the MexicanAmerican War (although those lands were sparsely populated).
The U.S. won't become larger.
It'll become another entity, called the "North American Community."
That is not a larger United States of America; that is something altogether different.