I also had this problem. I was so excited to have my son try on my first attempt at a sweater vest and then the first photo is what I ended up seeing! My giant-headed little buddy, how I love him.
Since I'm a pretty new knitter and this was my first sweater, I wasn't sure what to do. I ended up undoing some of the left shoulder seam and adding a loop and button. It works fine (FO on the little man in the second photo), although I've decided the button is too big. Looks like a chip on his shoulder!
Next time I'll know to
head off the problem!
Heidi