Landscape Memory Usage
For creating large terrains, Landscapes are generally a better choice than Static Meshes.
Landscapes use 4 bytes per vertex for the vertex data. Static Meshes store position as a 12-byte vector, and tangent X and Z vectors packed into 4 bytes each, and either 16-bit or 32-bit float UVs for a total of either 24 or 28 bytes per vertex.
This means that Static Meshes will use 6 or 7 times the memory of Landscapes for the same vertex density. Landscapes also store their data as Textures, and can stream out unused LOD levels for distant areas and load them from disk in the background as you approach them. Landscapes use a regular heightfield, so the collision data can also be stored more efficiently than the collision data for Static Meshes.