Sizing on this piece is a little funky! I watched some reviews that said it runs small, especially if you have a larger chest, so I ordered an XS (usual non-Ellie size) and a S.
Turns out they’re almost the same width in the bodice when laid on top of each other. I could see maybe half an inch of difference. On the body, it felt like the S just had a tiny bit more room for the bust.
Maybe I got an outlier dress, but in the end this ran TTS for me in my normal dress size. On me, TTS is snug enough to wear bra-less without feeling stifled. If you like a looser or more comfortable fit, consider sizing up.