Xport does not scale/resize images. Please refer the other FAQs on “Creating Flat and Scope DCPs in Xport”.
If your source file’s dimensions are smaller than the screen dimensions, you will have to resize the image externally before bringing it into Compressor/Xport.
If your file is in the HD 1.77 aspect (1280x720) - it should be resized to 1920x1080. This is a proportionate scale without any stretching of the image. Xport will then pad it with 39 pixels of black on either side to make it fit the Flat aspect of 1998x1080 while creating the DCP.