Crunches CAN be an effective tool, however there comes a point where they do more toning and conditioning than they do size building.
The function of the abdominals is to curl the torso "forward" and "in" (These are subjective to the position that you start in). If you take your lower ribcage and try to picture touching that to your hips, causing your upper body to kind of curl into a ball, that is the motion that works the abdominals.
If most of the motion is occurring at your hip joints and your back is relatively straight, then you are working your hip flexors rather than your abdominals as the primary mover.
Now, if your abs are underdeveloped, then crunches will definitely work the abs. However, once you get them to a base level of conditioning, then you don't really get much more out of doing more crunches. It's kind of like being able to bench 250, so you don't get much out of doing bunches of presses at 135.
Now if you add weight, vary the exercise (Such as hanging crunches), then you can get back to the building, but you are going to need to keep increasing the stress on the muscle to see advancement in size.
Addendum - As with almost every muscle group in the body, there are a group of antagonistic (opposing) muscles, and these are the muscles of the lower back (erector spinae, etc.). For all the work that you do on your abdominal area, you need to match that with work on the lower back. Imbalances between these two are often a cause of lower back pain, and can also promote poor form, injury and other like complications. Any time you work one muscle group, you need to make sure to have equal work on the opposing muscle group(s).