That's why when you're knitting clothing you need to make a test swatch first to make sure you have the correct gauge. You probably were a tight gauge knitter and needed to go up one size with the knitting needles.

You're certainly not the only person to have that happen. I occasionally post in a yarn arts knit and crochet forum and that just happened to a woman making a pullover sweater for her child so she had to unravel it.