In the currently latest version of the Gimp the default behaviour of the crop tool has changed to what they call non-destructive cropping. What this does is to apparently delete the visible pixels of a layer but not the change canvas size. This can lead to your cropped image being the same size that it was before the crop with blank bars around the cropped image. To restore it to how it worked in the past do the following.
- Select the crop tool in the toolbox.
- This opens the crop tool’s options.
- Click the check box immediately to the left of where it says “Delete cropped pixels”.
This should restore it to its previous behaviour.