I just finished making my first sweater, top down, in the round. It turned out great, just WAY too big. The cap sleeves hang down to my elbows. It turned out the “correct” size, I just apparently measured MYSELF wrong. Is there a way I can correct this without frogging the whole sweater and starting over? It’s 100% wool (cascade 220).
Have you considered Ben and Jerry’s ice cream for breakfast, lunch, and dinner every day for a week? 
Just kidding. Now I’ll shut up and let somebody post a constructive answer. 
First, you must consider whether you would rather end up with a completely botched sweater or if you would rather do it over. If you are willing to risk the former you can try partial felting. I have done this many times, but I do not recommend it. I strongly suggest regarding this as a learning experience and doing it over. For step-felting, really good wool will forgive you, but often the texture/hand of the yarn is so altered that … And once done, it cannot be undone and so if it doesn’t work well, it means a complete loss. Here is what you do: 1. Wash sweater in cold water 2. Roll in clean towels until fresh towels don’t get wet 3. Pad ironing board, bunch sweater down as close to desired size as possible and press with iron (through pressing cloth) at wool setting until nearly dry 4. Place in dryer at medium setting for 15 minutes (with some dry clothes) Repeat 1-4 to desired size. Now, after the first dryer treatment, you may still be able to pull the work apart and re-use the yarn, but not thereafter. After that, you have “felted” (“boiled-the-wool”) enough that it simply won’t come apart. Good luck.
I’ve done the same kind of sweater, using acrylic and the “correct” size and I chose to frog it back to the underarms where the pattern had me add 16 sts. under each arm. I looked like a big potato sack lady! I love the top down sweaters but they do come out oversized and are not usually fitted well. I made it from the acrylic first for this very reason so now I’ll know what to do with a more expensive yarn.
Smocking is an option as well, but may look “altered”.