Procedural terrain bigger than I set it, heightmap slightly off

Hello,

I'm about to procedurally create a terrain of the size 120x120. My problem is, the end product turns out to be 480x480

Also the height map isn't set right. That little edge isn't supposed to be there. The height map data is, to my knowledge set right. Here's a Debug.Log:

 heightMap[0,0] = 0.9363611 

That should be the top left corner of the terrain in the attached screenshot.

This is how I create the terrain:

 int chunkSize=120;
 float[,] heightMap = new float[chunkSize+1,chunkSize+1];
 // fill heightMap
 ....
 TerrainData terrainData = new TerrainData();
 terrainData.size = new Vector3(chunkSize, highest_c, chunkSize); 
 // highest_c is the highest point in my terrain
 terrainData.heightmapResolution = chunkSize + 1;
 terrainData.SetHeights(0, 0, heightMap);
 terrainData.baseMapResolution = chunkSize;    

 GameObject terrainObject = Terrain.CreateTerrainGameObject(terrainData);
 terrainObject.transform.position = new Vector3(start_x, 0, start_z);    
 Terrain terrain = terrainObject.GetComponent<Terrain>();
 terrain.basemapDistance = 100;
 terrain.castShadows = true;
 terrain.detailObjectDensity = 0.5f;
 terrain.detailObjectDistance = 40;
 terrain.heightmapMaximumLOD = 0;
     
 Terrain.activeTerrain.heightmapPixelError = 10;
 Terrain.activeTerrain.treeBillboardDistance = 100;
 Terrain.activeTerrain.treeCrossFadeLength = 20;
 Terrain.activeTerrain.treeDistance = 500;
 Terrain.activeTerrain.treeMaximumFullLODCount = 200;

Does anyone know what's causing this?

EDIT: Adding another screenie to illustrate one of the issues. What's that 'foot' doing there?