Since my bolero (Terry) is a S-M, and i wear an XS, it is a little big on me, mostly in the arms. It doesn't fit right over most short sleeved jerseys, but I wore it in Spain, a lot, when it was 60 in the AM and 95 in the afternoon, over sleeveless.
I like tight cycling clothes. I haven't thought of wearing it under a jacket. It might be OK under my wind jacket, which is looser, but definitely not my Gore winter jacket, which is very tight.