@Vidiot In the United states and 2 or 3 other countries this is the case in the rest of the world this is not the case. Besides that, Mormons count everyone who was ever baptised as a member regardless if the person still identifies himself as a mormon or not, and they also count all unbaptised children under 8 years old as a member.
So their numbers are inflated. On the other hand the number of Jehovah's Witnesses is considered (by scholars) to be conservative cause JW's only count those who participate regurarly in the preaching work.
That is why JW's say that there are only 1.2 million JW's in the U.S and national surveys say there are more than 2 million people in the U.S that identify themselves as JW.