The default return priority will send traffic back to these trades with less priority if you lower it.
If you have several BIG trades and some very small ones, then the best is to add some mid sized trades because if the gap between the big and the small trades has nothing in the middle (mid sized trades) then you may be losing traffic just because you don't have trades that could take the mid part of that traffic.
With several BIG and some small trades the small ones get their traffic returned pretty fast, so then the big ones round robing the return traffic between them. This usually ends bad because the traffic mix becomes very simple. specially if the differene is too big. For example, 5 trades of 10k each one and 5 trades of 200 uniques each one and no other trades. Adding several trades ranging from 1k - 7k will make the site grow almost exponentially till it reaches it's normal traffic value.
Of course, you can adjust the return priority and the maximum return percentage for the big trades, but then you will just starve the big trades trying to favor the small ones.
If you want to grow some small trade fast then let it on Overkill mode for several hours or a day. If you have enough traffic it will jump up pretty fast as long as the site you are trading with has enough traffic to return back when you set it on overkill