Front of sweater too big

Hi!
I have recently started knitting, I’ve made my way through some small projects, and now I have started a very easy sweater. The front and the back are identical squares, that are sewn together at the top, apart from a hole for the head. I have just finished one of the pieces, and to my horror, it is 10 cm. too wide. I really don’t get it, because I measured it a couple of times while knitting, I guess not thoroughly enough. Anyway, here is the question:

Do I have to unravel everything, and start over, or is there some easier/quicker way to undo the damage? e.g. make the “identical” piece narrower, and that way compensate? I suppose the sweater will look very strange if I do that, the sleeves will be in a very awkward position.

thank you in advance;)

Check to make sure why the second piece turned out larger - did you use the wrong needles, follow the wrong size, your knitting got looser? Once you’ve figured that out, the just rip out the one piece and re knit. You can just start from the bind off to reknit as you unravel it, you won’t need to wind it into a ball, so that will save you some time.