I wear a forgiving large but ordered an extra-large. I am a baby boomer with an approximate arm measurement taken around arm under the underarm, of 15" and underarm around shoulder socket of 17".  And this is tight.  I got the top on however it is tight and stayed tight all day. There's no easement between underarm and material. However there's plenty of fabric around the waist.  I did keep this top because I am 25 lbs. Away from my target weight, so I am rolling the dice on this one.