[Chris in CT]

I must say that I am a little confused. Can someone clarify, please? I had always thought that the issue was what the wall thickness of the brake drum was, not necessarily the inside diameter. Obviously, the I.D. matters within a certain tolerance range, but I have never heard of a brake drum being discarded because of an oversize I.D., only for a wall thickness that was too little. Educate me here? Thanks!
Chris