Have you got a link to statistics for the group?
They may have missed the boat in terms expanding into the west. JWs and Mormons had most of their success in pretty short windows of growth during the 1940/50s and 1970s. Most of the JWs I know came in, or their parents or grandparents came in, either soon after the Second World War, or in the lead up to 1975. (There was very little growth in the 1960s for some reason) And most of those who came in did so after moving to a new location and losing touch with family and friends. I don't think religions similar to JWs or Mormons would have the same success now as JWs did in the 1950s or 1970s.
If the INC collect tithes then I can well understand that their organisation has more money than JWs, even if their members are poor. JWs are in the pretty unique situation among religious organisations of having relied so heavily upon a source of income (publishing) that has completely dried up. In fact worse than dried up. Publishing has changed from being a tremendous source of income, to being a huge financial drain. They don't have a tradition of collecting donations and it's proving pretty difficult for them to persuade their members to begin contributing large amounts of money now.