I was trying to illustrate how different shapes take up different volume and can throw you off, making one meal a feast and the next a famine if you try to guess at a half-pound, but the picture didn't cooperate.  8 ounces of elbow mac, rigatoni and spaghetti.  Compare the elbows to the rigatoni.

Alton Brown said the same thing about weighing flour instead of using measuring cups, and since pasta is flour and water (except egg pastas), weighing is more reliable than eyeballing or using measuring vessels.