I have a OTTB.
My horse "fits" a semi-bar saddle. But with a built-up cut back saddle pad a full tree fits too.
If your horse is of average build and nicely weighted, try a built-up cut back saddle pad to take up the extra space that to wide tree fit creates.
If this works it opens up a entire new choice of saddle for you to try. You lose nothing for trying...
As far as length of seat...
Yes, it will make a difference in comfort to you and
especially to your horse!
When your body is squeezed into to small a seat that pushes you against/onto a cantle that should gently support, your cantle could instead be used as a compression point for overflowing bulk, creating excessive pressure on a very vulnerable location of the horses anatomy.
I'm sorry, I don't mean for that to be cruel but honest a comment.
From English to western conversion rule of thumb is 1.5" - maybe 2" smaller.
So, English 17.5" to me converts to
no smaller than a 16" western seat since rare to see 15.5 seats.
That conversion only works providing you are riding in a properly sized English saddle to start with.
There is a sweet spot, every saddle has one, the deepest part of the seat ...
You sit deep in/on the wrong spot because you are squished...you bet you can sore your horse real quick.
You ride in to small a saddle your balance is also going to be thrown off and that
is a issue on the horse, safety while moving and added stresses to his entire body. That makes a issue for you too..
Have you considered looking a bit larger seat instead of smaller? It is only a number...truly.
If your horse can handle a 16" size bet he can fit 1" more...need to make it shorter, look at round skirts versus square just saved you space!
Don't be someone who is number conscious cause no one sees numbers when astride..what you do see though is someone squished in to tight a seat and the horse paying the price.
Make sure you are sitting in the right size seat...if looking used, go sit in many different styles of saddle design and sit in a variety of manufacturers and seat sizes.
They
are all different, rarely are any manufacturers the same unless using the same tree base for their saddle to be constructed on.
I found you 52 saddles of various make, tree widths, all used and under $1000 maximum located in many states and areas with 2 clicks of my mouse...
https://www.horseclicks.com/western/A/saddles-for-sale?ads_pid=22&ads_lid=3&page=1
A bit of location shared would give some here a chance to help you search...many love to "shop" for others.
But what you propose,
no...
No to to small a seat for you cause it
is going to affect the horse over time astride...something you obviously are trying hard to avoid.
This is a sizable investment.
Get what is right so you over time are happy and comfortable, which in turn makes the horse most comfortable too.
:runninghorse2:...
jmo...