Are the seat sizes of saddles different between each brand? More specifically between a Trilogy and Toulouse? When I bought my saddle, the Toulouse, the girl who fitted said I was a 17.5. I ride in a 17.5 Trilogy saddle on another horse and my trainer says I am swimming in it. I don't know if the results of my weight loss could be the reason why the 17.5 is too big or if the Trilogy just happens to be bigger than my saddle. I don't keep my saddle at my trainers since my horse is leased out and I won't be out there until next week to have my trainer take a look. If it helps, my jumping saddle was a 17 so I don't know why my dressage saddle was bigger.
I use a size 18 in a lot of saddles because I have a long leg...I am of 5'5 and 130 pounds so a lot of fitters think I need a 17'5 but only in the flatter saddles can I ride in the 17.5. I am more comfortable in an 18 Hennig but fit a Black Country 17.5 fine
Sit in them!
Humans don’t mind duress, in fact they thrive on it. What they mind is not feeling necessary. –Sebastian Junger
What the measuring tape says isn't necessarily a good guide to saddle fit and feel. A flatter seat will feel roomier than a deep one. Size and placement of thigh blocks will affect fit and feel, too. Leg length, size & shape of pelvis, rider weight are factors. Bogey2 is right. You have to sit (actually, RIDE) in them to figure out what works.
If it helps, my jumping saddle was a 17 so I don't know why my dressage saddle was bigger.
Dressage saddles tend to be deeper, so to have the same amount of room for your hiney, the seat size is often larger. It really comes down to the depth of the seat, as well as how much room there is at the bottom of it - you can have deep with a flatter-bottomed U and use a smaller size, or deep with a very small seat area and need a bigger size.
As for the swimming issue above, no, there are no standards. A given saddle is measured from the front button/nail to the middle of the cantle. But, there is no standard placement of that button.
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Not only can it differ between brands, it can differ between models within a single brand. If can also vary between two saddles that measure exactly the same on a tape measure due to differences in seat architecture.
As a simple example, my dressage saddle and my jump saddle are both 17.5's. Most people would say that that's "bad" because I "should have to go up 1/2" in seat size to account for the high cantle and deep seat on the dressage saddle." But because of the relatively long working center on this dressage saddle, I have just as much room in this deep-seated dressage saddle as I do in my jump saddle with its medium deep seat and forward balance point.
The Trilogy dressage saddles almost universally have long working centers, so they tend to be pretty roomy.