这件事说明一个道理，那些能被定罪的都是苍蝇，犯的案太小，那些大老虎犯的事太大，大到难以取证，所以就不会被抓了。
This explains the fact that those convicted are flies, that the cases are too small, and that the big tigers are too big to be able to testify, so they will not be arrested.