kairos: I remember in my area we would have two distinct convention dates per year then they changed it to 3 or 4 with of course smaller amounts of people at each one. By doing this and saying that some would be assigned to other areas altogether there was no way of telling how few were still going.
Not only that, but smaller groups mean more control. Smaller assemblies mean that there is less chance of those groups communicating with each other about 'apostate' matters.
Breaking down large groups into smaller, more manageable ones, is a tactic that has met with much success in the past. Rutherford found that out when he went to Germany way back prior to WW2 and tried out that method of control. Re-structuring and re-branding is not new to the WTS.