According to Mrs Chang the first flushes are likely to release small stones which were in the front of our gb. Later we start to push out the old stones that were in the back and can then be released. In my last flushes(I´ve done 4 so far) I had some stones of 3cm long. I myself could not believe it!!(I now understand how a chicken feels when they lay an egg!!)
However, it may be the case that you don´t have large stones st all, but in general we do.