Why is it important for members of the UN to have a smaller body,?
Its not, because thats not how the UN works. Yes, the General Assembly deals with a broad range of issues, but thats because the General Assembly deals with all issues that can be raised to it. The security council, on the other hand, is simply there to ensure the juggernauts (or former juggernauts) cannot unilaterally infringe on the interests of the other juggernauts at the table using the excuse of by popular demand. It doesnt really deal with threats to international peace, except in the sense that the major threats to international peace are all in the security council, and their very presence in it shackles them and limits their options to threaten international peace.