Sizing problem

[pbs]

I'm knitting a jacket/sweater for my 13-month-old grandson--it's a rectangle back, then 2 fronts, then 2 sleeves. I'm using the size 18-24 mo. size, but my rectangle back if exactly 1" narrower than it's supposed to be. I know I'm a tight knitter, but I swear I swatched it--actually I thought it might be a little big. I really wanted him to be able to wear it at least 1-1/2 years, but his t-shirts are 12" wide right no