Lengthening

I made the music box (view B - no pleats) in a size 6 around and 7 length. It really fits well over all, but I notice it is slightly tight around the arms (my daughter hasn't complained, but I notice it when I zip it). I only lengthened the skirt. I wonder if it would work better to also lengthen the top portion? If so, where do you add the length. Do I cut the paper tracing at the top of the arm holes, and then add some length to them? How much do I add? I am getting ready to draft out my patterns in the next size up, so I need to get this all figured out. It takes me a while to get the jumpers sewn, working full time and all!