It could be the yarn, some of them stretch and grow. You should wash and block the gauge square the same as the FO, and even hang it when dry to mimic being worn. Or you may be knitting the wrong size. Many people overestimate their size and think a sweater needs to be at least their bust size or larger. But if you measure one that fits you the way you like, you may find that it's actually a little smaller than you measure. That's called negative ease and a lot of tops look really good when knit with about 2" negative ease.
sue- knitting heretic