There is a reason short boats look wide, other than "Because they are." Boats float due to displaced water volume. If you make a boat shorter you are reducing its volume. So it will sink deeper in the water. This is OK, up to a point.

But, sitting deep in the water has downsides, for example it can make it harder to turn. To make up the volume lost, without sinking, you must make the boat wider. So, short boats are wide so they will float right.

If you combine this with the fact that relative to their link even a narrow short boat, will be wider in proportion, short kayaks tend to look stout or tubby. It just goes with the territory.

That said, shorter waterlines are generally more maneuverable. But it is important to look at the waterline length, not the over all length. For example, the Guillemot is 17' long but its waterline length is 14.7' long where you look at the Wilderness Systems Focus 150, the waterline length is probably 14.5'. So, despite being 2' shorter it is only a few inches shorter at the waterline.

Dan has mentioned rocker. A design like the Guillemot has a lot of rocker, where a design like the Little Auk has less. Short boats often have fairly low rocker because paddling a nearly round boat is without some tracking designed in.

So most short designs may not have a huge advantage in the maneuverability department over a boat like the Guillemot.

With all that in mind, I like the 14' range for kayaks. It is long enough to be reasonably fast, allows narrow enough width for good looks and efficient paddling, is short enough to have good maneuverability. Obviously, every design is different, but 14' is often a good length.