You are very right, the only solution to bounty hunters getting low rewards is what you've stated in your post, there should be limit to numbers of participants needed for specific bounty campaign, so that rewards will be fair for few hunters that manage to get in.
Bounty campaigns do not care how much each participant will earn. They allocate a pool for a bounty campaign and it makes no difference to them by how many participants divide it by 10 or by 100. On the contrary, the more participants there are, the more noticeable the advertising on the forum will be. You yourself can see how many people are already participating in the campaign and decide whether it makes sense to participate because the reward will turn out to be small.