What is the difference between seat size in English/Western saddles?

Is there a difference in seat size in English vs Western saddles? Meaning, if I use a 17" English saddle, would I use a 17" Western saddle? Or are they not the same seat measurements at all?
5'9", 140 + or -, female.

I ride in a 17" english saddle and I have a 16" western saddle. I am 5'8"

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What is the difference between seat size in English/Western saddles?