Fitting CAIR saddle on downhill horse

But I was told by a few triners to get a bigger saddle. Because according to them, according to me, when I think about it, and accoding to how the mare isgoing, better to have a saddle inch bigger, but fitting, and allowing me to sit well and not to disturb her on her back. Than to have a smaller saddle, giving my weight on smaller area and not allowing me to sit properly, disturbing her all the time etc... I must say she is going very wwell in 17" saddles. I tried 17,5" and that was already pinching her too back backwards in her spine, but 17" works well for her...

A lot depends on your height too
I ride 15.2's that are probably also bigger built than your pony in 16.5 inch saddles but I am only 5ft 3
The man I used to work for was 6ft 2 and rode a 16.3 horse in a 17.5 inch saddle
Saddles also have width fittings
If your saddle is sliding so much that it needs something to keep it still then something is wrong somewhere

You might also try an anatomical girth; that is what I had to do with my Morgan mare who has a big belly (and I had the saddle fitter adjust the saddle to fit her).