I know skinny t-shirts are a thing, but there are times when they are too skinny! My eldest has a favorite print shirt that fits this category, and drives her crazy when it rides up. We did look for a larger size, but it has been a few years since she received it as a gift. To fix it, I found a thin knit fabric that matched the weight of the shirt (and coordinated with the print), then picked out the side seams all the way up, including the sleeve seam. Using a different, well-fitted shirt as a guide, I cut 3” strips of the new fabric and sewed them in using the jersey stitch on my sewing machine.