Yep, I have a 17" medium on my 3/4 tb and it fits her really well. She has got quite big withers...will try and find a picture so you get an idea of her wither height compared to her back. Might show it a bit better on the 2nd one.
with her ideal jessica on:
I love my dressage saddle, definately worth trying. My saddle fitter actually recommended I try the Jessica because she thought it would suit the shape of her back.
i've got an Ideal Suzannah which was fitted to a very high-withered tb, if that helps. it did have special panels to fill in the gaps because the saddler said the horse's ribs started very low down, if that makes sense.
I have an Ideal Josephine (Jessica tree) on my high withered one and it's lovely - also fits the low withered WB funnily enough - think these saddles fit anything
I have found them very versatile and have used mine on a large variety from ponies through TBs to warmbloods. I find they sit well on the horse and they seem comfortable in them.