It seems that your image / layer (named frog enhanced) has not the same ratio than the background, thus if you keep the aspect ratio (with the link chain) it will never be 7200x10800, one side will always be longer or shorter
One question, though: What the white layer in the background for?
Out of the box, solutions that come to my mind
1 rescale but unlink the ratio by unticking the chain (image will be slightly distorted, you might not notice a real difference at those size)
2 rescale with ratio but after rescaling go to Image ➤ Fit Canvas to Layer
3 rescale with ratio then after rescaling go to menu Layer ➤ Layer to image size ➤ this will cut excess outside
Also when you use the scale tool be sure that the right option is selected (red arrow is for layer, yellow arrow is the full image)
One question, though: What the white layer in the background for?
Out of the box, solutions that come to my mind
1 rescale but unlink the ratio by unticking the chain (image will be slightly distorted, you might not notice a real difference at those size)
2 rescale with ratio but after rescaling go to Image ➤ Fit Canvas to Layer
3 rescale with ratio then after rescaling go to menu Layer ➤ Layer to image size ➤ this will cut excess outside
Also when you use the scale tool be sure that the right option is selected (red arrow is for layer, yellow arrow is the full image)