PTA.S1.Q03 - In a democratic society

Can anyone give me a rundown as to why the answer for this one is "D- A political interest group can become ineffective by expanding to include as wide a membership as possible" and not "A-political interest groups are generally less influential when their membership is expanding than when it is numerically stable?" Thanks!

Admin Note: Edited title. Please use the format: "PT#.S#.Q# - brief description of the question"