Well, let's look at two hypothetical sets with five creatures at cmc < 5 each.
Set 1: one 0/6, two 2/2's, and two 2/1's.
Set 2: one 2/4, one 2/3, two 1/2's and a 2/1.
Now, simply looking at the numbers, I would call set 1 an aggressive set. There is one big blocker, but it can't actually kill...