I think she has broad shoulders/a larger frame. I believe I mentioned in this thread before that I once bought a jumper by &otherstories she had (of course she has sold it by now... the jumper was from the flat era). She mentioned she had size M so I ordered size M as well since I figured what fit her ought to fit me as well and I usually fit M. To my surprise it was too big on me (I´m rather average in size with a slightly larger chest, which fits my proportions though) so I exchanged it for an S which looked the same on me as a M looked on Anna. (I think my issue is that I see myself as "bigger" than I really am since no matter how much or little I weigh I am always a bit round/chubby - or as my fiancé likes to call it: "soft"
- even though my frame as a whole is not large)