Different religious groups have different ways of counting membership. JWs have one of the strictest measures of any religious group and the Mormons are at the opposite end of the spectrum claiming many more members than reality. Scholars interested in establishing some sort of fair comparison between groups have used censuses to evaluate membership totals. The result in the table below compares the number of members claimed by the group compared with the academic estimate of “adherence” on the basis of census results.
The figure of 15 million for JWs makes sense because if asked to estimate the number who consider themselves to be JWs then a figure in between publisher number and the Memorial attendance sounds about right because many inactive JWs still consider themselves JWs but not everyone who attends the Memorial. The Mormon figure of 7 million also makes sense because it’s widely known the Mormon church grossly inflates their numbers by counting everybody they’ve ever baptised even though many of them never return to the church after rushed baptisms.