what is the difference between terrorist group size and there effectiveness?

Answer 1

Simply physics.

Let's say Group1 has 1000 people, all age 80, with no training and no weapons. Let's say Group2 has 20 people, around age 25, all with five years of military training and modern weapons.

Which is going to be more effective? Generally Group2, because they can bring more force to bear.

Sheer numbers are rarely sufficient to accomplish anything. You need people who are active, productive, and capable of doing what you need them to do. Plus, you need efficient communications, which can often be more easily accomplished with a smaller group than a larger one.

The same rule that defines how effective an army or a business company would be also applies to any other endeavor, even terrorism.