AssetBundle size

Why is a single 512m x 512m terrain with a prefab size of 562kb and a terraindata asset size of 4859kb, build into a compressed assetbundle of 98,879kb? (This terrain has no details, detailprototypes, treeinstances, treeprototypes)

How can I get an terrain of under 6Mb into an assetbundle of less that 100Mb? What on earth is it saving, and how can I trim things down?

Materials, Textures, Shaders…