As heathen suggests, for more insight into the often counterintuitive 3-way relationship between microphone pickup pattern, angle and spacing with regards to a stereo pair of microphones, read Michael William's Stereo Zoom paper a few times. It tends to take a few reads to really sink in.
As for a basic explanation to your question, the image doesn't narrow as the mics are angled further apart, it grows wider. At the same time the Stereo Recording Angle (SRA) narrows. The confusion stems from the difference in definition of stereo image and Stereo Recording Angle.
As you increase the angle between a pair of directional microphones, you increase the difference signal between channels, which results in a wider playback image- it sounds more spacious, and you hear a greater difference between left and right channels. This is analogous to zooming out with a camera lens- the cropped limit of the overall scene grows wider as you collect light from a wider "seen" area. But at the same time, the subjects themselves within the scene appear smaller. So if you're photographing say, your brother and his wife standing on the beach, and zoom way out, the overall scene displayed in the resulting photograph grows wider, but the portion of interest (your brother and his wife standing center frame) appear smaller, and take up a narrower portion of the photo. If you zoom in, that portion of interest effectively "expands" to fill the frame, even though, rather because the overall scenic perspective has grown more narrow.