Why believe JW numbers? Because censuses confirm them and then some. Because academics have studied them and found them to be underestimates of JW numbers. Because I've seen attendances at various KHs and assemblies with my own eyes and they look similar to the 1990s.
They are probably short of elders and making efforts to boost numbers.
Adventists are indeed growing, they are one of the few churches that are growing. Most churches are declining severely. JWs are staying about the same in the UK or growing slightly because of immigrant largely from Africa. In other counties they are still growing. In a few countries they are declining such as Poland. Congregations may be closing in some places, a mixture of fewer numbers but also as a money saving exercise in some cases.
Check out this comparison of JWs, Mormons and Adventists here:
http://www.sdahistorians.org/uploads/1/2/3/6/12365223/lawson_2_reassessing_size.pdf