I saw something about international/intercultural transgender issues, and apparently tomboy or tom is their term for FTM. However, I'm not sure if this means that much. I get a binder from a company in Taiwan, and have to get a pretty large size ,so it's just likely they were rather flat to begin with. I bought them for comfort and not that they bind better. Plastic is going to be very sweaty and not breathe. I suppose that's the point, but I don't think it's very good for them either to be honest. You'll lose some water weight, in other words sweat like crazy, but I don't think you can JUST lose weight off the chest, and the weight you lose if you want to do such a thing is going to be from all over your body (and not that significant anyway).
If they were here I'd tell them not to do it either. I am guessing these are small chested guys.
--Jay