I wasn’t referring to any other part of the saddle, just the adjustable plate. Any saddle with wool flocking can be re-flocked to tweak it to the horse’s shape, with more, or less full panels for example. But that doesn’t change the overall shape - won’t make flatter panels more curvy, or vice versa
I started lightly riding my WB the Summer of his 3yo year. We started in a M Bates Caprilli. Over the next 2 years, maybe 18 months, we progressed to the W, and it was with the XW plate that the shape no longer fit him. By the time he was around 7/8 or so (it’s been a while), he was in a 39cm Prestige dressage saddle, which is substantially bigger than that XW plate, and if he’d stayed sound, was going to be in a Black Country Quantum X (hoop tree) in an XW, also wider than the Bates
“very snug” needs more info
Yes, you want even pressure front to back, though some, mostly in the Dressage world, require saddles to fit with a very slight bridge so that there’s room for the horse to lift his back into it.
But there’s a difference between “snug” to the point it’s a great fit and it just needs a regular pad, and so snug that anything more than a baby pad makes it too narrow.
Why is your trainer not a fan? Are they also a saddle fitter? Reflocking a saddle is an option, but you really need someone very competent at that
Definitely not someone I’d be taking saddle advice from if that’s what she thinks. I don’t disagree that there’s a bit of a gimmick to it when the maker says “adjustable to fit all horses”, because NO saddle can do that, no matter how much you reflock it or widen/narrow it (which can be done on some saddles by a professional, it’s not a changeable gullet deal).
I, personally, and many fitters I know (but not all of them!) believe in fitting 1/2-1 size wide and using padding to get the “snug” fit. Pads are “softer”, more forgiving, then panels are, and you can use varying thickness of pads as a horse gains/loses weight or muscle. Balance International is where I first learned about this concept, and I still prefer it 15 years later.