babies r us undershirts

RussianMommy

do these tend to run on the smaller side?? I ordered a custom undershirt/onesie thing from Etsy in a size 9-12 months. When I got it, it looked newborn size, even though the tag does say 9-12 months. It's narrower than some of the 6 month shirts I have. I  let it slide when I got it, but I'm kind of disappointed. It's been a few months since I received it, so I would feel silly messaging the seller now - but for future reference, are the sizes THAT much smaller??? do they stretch?