The JW criteria for defining "membership" is rather more rigid than that used by the LDS, or just about any other religious group. That is, they define as "members" only those officially reporting "field service" on a regular basis (however inaccurate those official figures may be).
However, what a person might enter on a census questionnaire is quite a different matter.
For example, on one occasion whilst myself engaging in the "field service", I recall meeting a couple who assured us that they, too, were Jehovahs Witnesses. It turned out that several years previously these two had undergone a "Bible Study" with the JWs. They appeared to believe everything they had been told by that pioneer couple who "studied" with them - although their level of understanding may have been rather limited. However, they were never baptised, and certainly never attended any meetings, let alone engage in that JW trademark activity, knock on people's doors! Yet, it is entirely possible that they (and all other such persons) would - if ever asked - describe their religion as "Jehovahs Witness". In fact, the first time I ever filled out a census form, I myself gave my religion as Jehovahs Witness - even though at that stage I was only somebody's "Bible Study" (and would have been lucky to have attended more than about three or four meetings at the Kingdom Hall).
I rather suspect that the data used by the Mormon / LDS church to compile its official membership numbers shares greater similarities to that collected in a census than it does to anything else.
Expressed another way, it you were to compare the official membership numbers of the JWs with the official membership numbers of the Mormons / LDS, then you wouldn't exactly be comparing "apples with apples". Those two religious groups apply completely criteria to define what it means to be an "official" member.