Well... not sure if this is what you mean or not, but in GIMP, I would first and foremost make sure your landmass shapes had their own layer, ie, just the outlines(even if you don't "paint" the outlines on the finished map). Do a selection OUTSIDE the lines inside area, invert the selection, and make that a path. Now that you have the path, go to the paths "tab" and export. In your new image (which is best to be a multiple of the existing one... at least to start) on the paths tab, import and set to scale to fit image(which is why we want the new image a multiple of the previous one), then path to selection and stroke the selection. Now you have enlarged your original and maintained shapes. Note that very small things in the original might be missed by the selection process, so you may have to play with it some to get the selection "just right"