If you mean regular background (not related to storyboard), then that can be because you use 4:3 background - that will increase width to fit widescreen, but will make it bigger and will cut its height. If you want to avoid this, don't use 4:3 backgrounds but 16:9, preferably 1366x768.
If you mean an image in storyboard, then that's because default scale of image is 1.0, but storyboard size is 853x480, if the image size multiplied by 1 is larger than 853x480, it will become bigger. You need to count exact value and edit it in .osb file. You may want to check "Wrong full-screen scale" section in
my guide if that's the case.