Girth dilemma - how much do Antares girths stretch?

Stupid question, need opinions please:
Got a new saddle and anatomical girth, guesstimated the size at 52" for my 17hh mare, as my 54" girth always seemed a little big on her, by that I mean that it does up on both sides to almost the top holes, so I figured I’d be safer getting a smaller girth… well, said girth barely does up on the loosest holes, and only just. My dilemma is that it seems that there is extra material, like it seems like it should be able to do up more, but my mare bloats, the girth leather and elastics are stiff, and the billets are extremely stiff as well.
Thoughts? It’s been ages since I’ve bought a new girth, and this Antares seems stiffer than others I’ve used.

Many thanks in advance!