I was concerned that the M might have openings that were too small for my 14E, 170cm, 81kg body. Some big box store brand nursing tops I have, don't have a wide enough opening to pull down the bra, get the boob out, and feed.
These, however, are perfect! No issues at all, and I also find that I can pull down that top layer enough that I don't feel like I need to use a nursing cover.
I stopped breastfeeding about two weeks after the t-shirts arrived due to a piranha baby, but the shirts are actually truely able to be worn as regular tops, since they don't look like nursing tops.