There are only a few things we know for certain about where the Jaredites were.
1. They left many dead bodies and empty cities in the land Desolation (north of Mormon's structure).
2. Their capital Moron was near the land Desolation.
3. The narrow neck of land at one time couldn't be crossed because of snakes.
4. After the snakes were killed, this area and the land southward was kept as a game preserve.
5. The Jaredite population was at least 2,000,000. They had stripped the land of all timber (Helaman 3:6).
6. It took them 4 years to gather for the final battle.
We don't know if they landed on the east or the west coast. The current thought is that the Olmecs may have been the Jaredites. There are many smaller versions of Olmec-like statues in Colombia and Panama, but we just don't know what that means.
During the time of Mormon, the Nephites became limited to the land northward (the land Desolation and north). The land was inhabited from sea to sea. Even this population was smaller than the Jaredite population.
"New Millennium Atlas" by Rand McNally