MacBook Pro 13 resolution problem?
This happens because the game is not widescreen enabled (but 4:3). This usually happens with ports of old games, from the times, when 4:3 (or close to 4:3) resolutions were common (like 640x480, 800x600, 1024x768, 1280x1024). Look for resolution settings in the game and change it to something closer to 16:10 and the black bars will be smaller.