It's all about the Ureter.

Kidney stones are hard, crystalline mineral material (calcium) formed within the kidney.

They generally cause no pain while in the kidney, but as they move from the kidney through the Ureter on their way to the bladder, they are likely to cause severe pain. (let's be honest, severe is much to mild - excruciating is more accurate).

Once they reach the bladder, the path is much larger and comparably pain free from there on out.

A guy in Hungary had a stone removed this year that measured 6" in diameter. I guess my 1/8" stone wasn't so bad. Then again - he didn't have to 'pass' his. They just cut it out of him.