The joys of the pressure jerkin.
When I went to get mine at the start of the tour as a brand new shiny co-pilot even the smallest size was too big for me and the doc's wouldn't issue one as they felt that the bladders wouldn't inflate fast enough to protect my chest in the event of a depress.
3 months later I went back, having put on 2 stone ( serious beer drinking and eating) and hey, presto I fitted. Never used it in anger though though did take it on det, just in case.