You need to call imrotate with the 'crop' option to preserve the size of the input image.
As for the negative xy, it is possible for a rotation to bring points in/out of the image field of view. That's the whole reason why imrotate has the 'crop' option to begin with.
Be mindful as well that imrotate's origin of rotation is at the center of the image. However, the BoundingBox coordinates are relative to an origin at the upper left corner of the image.