I have bought the small and extra small of these and there is quite a difference between them. The small is a little bit too big for me, so I got the extra small and it's tiny! There's quite a big difference between them. I could really do with something in between but can't seem to find anything. Having said that, the protection it gives you is great (far better than the individual cup-type chest guards, which I've also tried). It's good enough to make it worth putting up with a less-than-perfect fit.
For years I had a medium size one of these (the top half of my body is a size 16)and it's always been too small so I got the largest size there was and to be honest it's not that much bigger. It's about an inch higher and about an inch wider and the "cups" are only slightly bigger.The trouble with the extra hight is I keep catching my chin on it!! (I do kickboxing).HOWEVER despite the size issues I would not be without one of these they are invaluable. I've been kicked and punched in the chest many times and without wearing this chest guard the pain would've been awful!
Although they need to do something about the sizing for bigger ladies I still highly recommend buying one of these...I will not train or fight without one! PS Excellent value for money too.