Hi and welcome to KH. Did you knit it top-down? If so I think you might just rip it back to a few inches above where it is too snug and add some increases at the sides where seams would be if it were seamed. Decide how much bigger it needs to be, figure out based on your sts/inch how many stitches you need, and then put in a centered double increase every so many rows. If you knit it from the bottom, it's more complicated. HTH
I should never overestimate my basic arithmetic skills.