Tiff will not show full image saved

I have a file that I expanded so I don’t lose part of my photo to a mat but GigaPixel will not load the full photo in TIFF format. It is still the original size before the expansion. If I save it as a PNG, the full expanded photo loads. Very frustrating.

I should elaborate. I was having issues trying to use Gigapixel in Photoshop so switched to using the program alone, saving the file in Photoshop as a TIFF with no layers. This is where the issue was with it importing the pre-expanded version of the image. It worked without issue in the PNG file format, but given what I need it for, PNG isn’t going to cut it.