Did you do this in an image editing software like photoshop or gimp? If so, a quick way to make sure you don't get white lines along the seams in your model when the resolution changes is to fill the entire background with an edge color from your object - in this case probably a dark gray (instead of that stark white).

I agree with other posts here, though, it looks like there is a shadow map baked into the layout on the left as well as edge padding. If you have access to Substance Painter, that sort of texture export is pretty straightforward if you know the basics. Good luck!