Thanks guys! I figured out my problem was understanding the pattern as a concept -- I'm still figuring out what each piece of the sweater is supposed to look like unattached, so it was a big surprise when it turned into a mostly-tube with a bit of a cut running along one side. It makes lots more sense that way, lol. For some reason, the "turn your work" stuff is where I'd been stuck since knitting in the round is a big succession of turning the same piece over and over again. It never occured to me to turn it in the opposite direction and work on a purl row.
I think I just needed it rephrased so I could get over my mental block with it. I tried what you've all suggested (after quite a bit of frogging to get rid of the mess I made when I tried to do it on my own) and the first sleeve came out looking proportional, at least! Now I just have to make this work a second time for the other side, haha.
Thanks so much for all your help!