I agree too. It just sounded different in the initial post. My upper arms are 12 inches. I am also an ecto morph and small boned. But, I feel that my arm isolation stuff helps with the larger muscle group exercises. I can tell you for sure that my pull-ups made some enormous gains when I did curls in addition to upper back work.
I believe in doing 3 different exercises for large muscle groups and only 1 exercise for the smaller "helper" muscles.
Setting a size (like 15 inch arms) was the problem I had. I have a freind who has 16+ inch arms who can't do as much weight as me on curls or pull-ups. But, he is a mesomorph and very big boned. So, I thought the 15" measurement to be a bit general. Also, I find that tricep work can improve your bench and military numbers.
But, I strongly agree with the fact that compound exercises will work smaller groups.
BTW - you can check your bone size by bending your arm to 45 degrees and then using a caliper, measure the protruding bones on each side of your elbow.
-Rip