Personally, I went by the resistance sheet but then was fairly brutal to rabbit tribe in terms of Watcher operations to reduce that number somewhat. At least until the PCs started drawing attention to the Resistance proper.
That way I kept the feeling of oppression and desire for action going among the players, without culling so many animals that I completely crippled a tribe.
They were still the largest tribe after 5 months but only just, and pretty much irrelevant to our story, as the players felt a bit traumatised by the suggested event there and stayed well away after that. It was brought during Mechatron last week in fact, as an example of how mean I and/or Mutant can be.