How to reproduce:
1. Open the user attached project
2. Go to "Builds" --> "Build Asset Bundles" (custom menu items, to build asset bundles)
3. Compare built Asset Bundle size and Terrain asset file size

Result: AssetBundle size is around ~100kb bigger than Terrain asset file

Reproducible with - 2018.2.0a1, 2018.1.0b9, 2017.3.1p2, 2017.3.0a1

Note - Both Standalone and Android as Asset Bundle build targets produce similar results

By design --> So the Terrain Asset in the Assets folder contains the TerrainData (hight map data) and the splat map (texture painting data) and references to things this terrain needs such as textures, materials, and shaders. Building this asset into an asset bundle causes all of the dependencies to be pulled into that bundle. This includes the textures used to paint the terrain, all the shaders needed, and materials. This causes the final size of the terrain to appear much larger because you are looking at the full set of dependencies, not just the minimum data needed.