Size differences....UK/USA

[mower]

Ok....we all know boots are a half size bigger over here but what about uniforms?
Recently i purchased a mint '71 hot weather flyers jacket...size small/long
a very nice purchase for the price....but......it would only fit a 15 year old....ok i now we all are lot bigger these days but my poplin third pattern jungle top(small/regular)fits brill.....so wats going on with sizes....seeing these suits were meant for the army,as the airforce and marines had their own nomex flight suits,why do the sizes differ so much??
Has anyone any answers on this.......help

[spc4lipton]

Have learnt over the years that if buying military items of clothing both British and US by mail order that its a bit of a lottery as half the time they bear no resemblance size wise to whats printed on the size labels!!!!!!

Sometimes fit much bigger than stated sizes but unfortunately also smaller. In jungle jackets i take a small/reg but a lot of times no 2 of these can be the same size and asking a seller for actual measurements doesnt allways work either-pretty much hit and miss im afraid!!!