The correct answer is what Frank explained to you. You somehow exported your shirt with another uv-map you probably created while trying to bake.
Slightly alternative approach could be, UV-Map the shirt as before (the front must be one piece), export the map as pdf, fit in the picture in your image app, have the same background for the whole pic (the color of the shirt), make sure the plane (or level) with the uvs in your painting app isn't visible, export as jpg or png, load pic in Cheetah in the material as diffuse, test-render in Cheetah, when everything is ok, export to Unity.
This way you don't have much to do with downscaling, baking or whatever.
Slightly alternative approach could be, UV-Map the shirt as before (the front must be one piece), export the map as pdf, fit in the picture in your image app, have the same background for the whole pic (the color of the shirt), make sure the plane (or level) with the uvs in your painting app isn't visible, export as jpg or png, load pic in Cheetah in the material as diffuse, test-render in Cheetah, when everything is ok, export to Unity.
This way you don't have much to do with downscaling, baking or whatever.