During my time with the JWs (1969-1994), obesity amongst them was not that common. Furthermore, the few that did have an issue with their weight tended to be those that had thyroid or similar problems.
In the years since then, the only contact I have had with the Witnesses is when they have called at my door, or when I have seen them manning the Apathy Cart. On neither occasion have any stood out as noticeably overweight.