Very bulky seam. The problem started with a strange sleeve configuration, with what seems to be too many decreases so that the edge of the sleeve has more steps in it than the armhole opening. You can see this in the picture below: notice where the seam between sleeve and body veers from a vertical line to an angled horizontal toward the right?
I used the Malabrigo to sew up the seam. That worked fine on the side seams of the body, but it seems to be too heavy for the sleeve seams. Also, I followed the directions for finishing the sweater: sew the sleeve into the armhole, then finish the seam of the sleeve. Usually, I reverse the order: sew the seam of the sleeve first (the one that runs along the inside of your arm) and then fit the sleeve into the body. That's the way that you do it when you sew a garment and that's the way I learned to put clothing together in seventh-grade home ec.
Second go around: I used a mercerized cotton 10/2 thread from my weaving stash and sewed the sleeve seam first. Better, more flexible, not as bulky. But when you stretch the sleeve a bit - which will happen when you wear it - you can see the thread. And it doesn't look good, even though I remember that sweaters that I bought years ago at an Italian flea market were a bulky yarn finished with ordinary thread.
Perfectionism and the learning curve strike again. After lunch, I'm going to redo the sleeve seam with some left-over Debbie Bliss Cashmerino Aran from the Fetching mitts, then insert the sleeve into the body and try to work the sleeve seam with less bulk.