I have a ton of these shirts & I love them. I wear a 40DD, size 18, so I'm bigger than they recommend but they aren't stretched or strained across my chest. The only place they are tight is in the arms.

To make the arms more comfortable, I first measure my arm & cut two pieces of ribbon about 4" or so larger than my arm measurement. I turn the shirt inside out, and make two small snips on the inside of the channel where the elastic is run. One on each side of the center seam where it's sewn in place. Pull the elastic out one side, safety-pin or tie the ribbon to the elastic, and then cut the elastic above that point as close to the center seam as possible. Pull the elastic out the other side & if all goes well it pulls the ribbon through as I pull the elastic out. At that point, I just tie the ribbon where it's comfortable, trim if necessary (or not if I want to be able to adjust it later) and no more too tight arm holes! For longevity, ideally I should probably hand sew the little cuts with a whip stitch to keep them from fraying, but I haven't done that yet.
(if the ribbon doesn't pull through with the elastic, I just get rid of the elastic first & just use a safety pin to inch the ribbon through.)

Careful of the dye in the wash, don't put them with other light colored things. I always hang mine to dry, I never put them in the dryer. The dye issues would annoy me if the price wasn't so reasonable.