@Jalyha: You should export the map, gimp's xcf is not an actual image, but gimp's impression of it.
Go to File > Export (or shortcut Ctrl+E) and save your map as something.png. And now you have a displayable version of your map with original quality.

Alternatively, to reduce the size, before exporting go to Image > Scale Image and change its size. Just be careful not to save it after scaling, just exporting, otherwise you will permanently reduce your map's resolution.