I haven't really studied the level generation alogrithm much, but AFAIK it first tries to place all the prefab rooms for the level and force-aborts if unable to do so, then places some random rooms and generates all doors, then connects the doors and other square with required attachement by tunnels dug in the shortest way that doesn't intersect any room. There is no code to create wider tunnels - they happen randomly when two tunnels are created next to each other.