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the lighter gray area is the uv active area, you can zoom in as well, keep all the UV to that area,
to make everything fit there, just find the menu polygons and select layout, a tip is to click on the box button in the side and configure it.
The other areas can be used to make multiple UV areas used fot animation.
This might be because the texture has a wireframe in it outlining the UVs
This wireframe is nice if you want to see whats going on when painting the texture, but you have to remove it before exporting the texture.