Resizing Terrain without affecting the existing layout

Hello, I was just wondering, if there was any way to alter terrain without effecting what is already there, I've tried to use photoshop to add on colour to the outside, but it just destroys quite a lot of the terrain instead. (And resizes it) I made this test scene.
Here's what it looks like 50x50x600 (Normal):




Here's what it looks like at 100x100x1200:


The Normal RAW image looks like this:


And the edited one looks like this:





So yeah, any suggestions would be greatly appreciated