This new model runs smaller than the one I bought last year. The Size 3 from 2008 fit my huge 17h Warmblood just great and is a bit loose on my 17h narrow TB. Granted, it stretched out some but it is 3-4 inches longer in the elastic than the 2009 model. The size 3 that I bought this year is really tight and I didn't trust it to stretch out enough. Sent it back =(
So, yes they do stretch, but they do seem to be running small. May consider a Size 3 if you can get your hands on one.
I have the Nunn Finer elastic breastplate and it did stretch a bit. I think they are a little bigger then the Antares, I'm using a standard size breastplate on a huge WB, although it is on the last hole around the neck.