Our tanks are true to size. If you have bigger boobs, like a DDD or more, I would size up. It won't fit that different, and it just gives you an extra bit of coverage on the boobs. The model in the photo shoot is wearing a small, and the model wearing the bra and bloomer set is wearing an XS. Here's a chart:

 US size UK size Buttercup size
0 - 2 2 - 4 XS
4 6 S
6 8 M
8 10 L
10 12 XL