Much talk has been about whitespace or copyspace in images - designers prefer it and photographers like to emphasize their subjects with it.
Whitespace should not be white. Why? Because if it is white, it is just empty. Empty space that makes the file (look) big, but in reality only makes the designers pay more for nothing. If you isolate your shots, then you should crop tightly around it, making the image as big as actual information in the image and not more. It seriously is not fun to pay for XXL size, if the usable image in it is only worth Medium credits.
Reviewers take it into account as well, but it is really the education for contributors, in order to avoid ill rejections.
If there is an uniform tone in the background, we advise to treat it the same as white, since any uniform background is relatively easy to extend to either direction if needed. If the background is complicated though, or on a gradient tone, including this extra space might be justified, but only to the extent of the actual frame of the camera.
Otherwise it is plain cheating, and cheating is not nice, you know ;)
Search for "copyspace" resulted in 94,000+ images and the majority of them are over white backgrounds. Most of them have moderate excess space, but there are some extreme examples as well.