I've actually looked at J1A-GT size chart vs J47A. From what I found, it seems like the shoulder and sleeve measurements for a medium J47A is the about same as a large J1A.
But if I remember correctly, the shoulder measurements on the chart is the length from where the sleeve begins to the collar. So it doesn't really say if the overall length of the sleeves when I'm wearing it will be longer if I size up from a medium J47A to a large J1A.
Wondering if anyone who has both for a reasonable amount of time can comment on the difference.
Edit: another reason why I think the size chart is confusing is that I've read posts of people saying the J1A is roomier around the chest but the measurements I found is 59cm vs 61cm for J1A vs J47A