ORIGINAL: polstery
larger props are more efficient meaning it will product more thrust at a given rpm which means more speed.
You might want to look into the relationship between prop diameter and prop pitch.
Larger props will not necessary give you speed if they don't have very much pitch. Take a 24" prop for instance. If the blades are totally flat (no pitch), then you won't go anywhere. Pitch is what determines speed, not diameter.