Has anyone knitted this sweater? It's a Black Purl pattern - a baby sweater knitted with sock yarn. I've been working on it since early summer; I have all the pieces knitted and I've joined the front and back at the shoulder with a three needle bind off (that's a very cool technique!

YAY ME!). But now I'm stumped.
I need to "backstitch the sleeves to body, matching the shaping and easing to fit as needed. Weave together the side and sleeve seams".
Never mind "easing to fit as needed"...my sleeves seem so much smaller than the arm holes on the sweater and I'm STUCK! I thought at first that I attached the front and back pieces incorrectly, but the they match exactly so the body looks totally right.
I won't even mention that I can't figure out the backstitch because that's irrelevant if my sleeves are wrong!
The baby is due in less than SIX weeks!! I need to get busy.. I'm so frustrated - - how do I get past this??