The only real advantage being small has is that a larger opponent might underestimate you or just have no idea how to fight someone smaller. Otherwise, in any physical fight, they're just going to have an advantage.

Why would they let you get to their belly with their superior reach? They can use their own knife/whatever to hit you first.

Obviously, for anything, they might also just not be as trained as you or have whatever other disadvantages, but in general, assuming equal skill and effort and no mental biases, i.e. everything else being equal, larger simply equals better in a physical fight.