I would consider myself to have average sized arms ( I normally wear a medium in shirts because I have broad shoulders and am tall), but after reading reviews on here, I bought a size up to make sure it was not too small in the arms because that would drive me crazy.  Well the arms fit great, the rest of it was huge and unflattering.  It has some weird design in the back and just looks terrible and boxy when on.  This one is going back for sure.