Definitely is stretchy! I feel like its not a medium either...i got pretty big with my 9lb baby and this fit me the whole time. I measured from seam to seam across the bust line and its 22". Ive never measured a shirt so forgive me if its a bit off My bra size is up to 38D and it still fits if that works better :)
If u look at the v neck in the 2nd pic it is sewn again at the top to hold it closed and open in the middle of the criss cross of fabric (just a little) It allows for a even more stretch for big tatas haha Or could possible even be unstitched if you wanted to open up the neck line and wear a cami underneath