There are two reasons why this happens.
The first possible reason is because the background color is the same as the border color. The image below is an example. The first image is the original image. The second and third images are both edited versions (white background). Even though the third image has the same border size (50px) as the second image, it looks similar to the original. This is because the color of the border is the same as the chosen background color (in this case, white).
The second possible reason is because of the border size and how it affects the product size. Product size simply means how large your product appears in the image (in this example, how much space the chair occupies within the image or canvas size). If the border size you've chosen is smaller than the background space in the original image, you will not see much difference in the final output.