Because tee shirts are stretch they are easy to add panels to. Unpick the side seam and the sleeve about half way to the shoulder seam on either side. Cut out a panel from fabric similar to the fabric or in a contrast. Allow around 7.5 cm (3″) at the top (slightly curved for the armhole, and make a little wider as you come down the panel. Because you have unpicked the sleeve, you can ease the new panel into the sleeve. I have done this with a few tee shirts that I bought but found they were too small. (I have a big bust) Most of the clothes I wear are black, so I had no trouble matching the black stretch fabric. If you want to have a contrast, you can sew some of the different coloured fabric onto the front of the tee to give it balance.
Happy sewing
Judith aka genie
Posted in Clothes too tight - Let out