The photos of the saddle you bought look fine - but actually that is the catch. Why was a saddle which will cost hundreds of dollars so cheap?
There should be a makers label somewhere under the flap - what does it say?
Measure from the button up by the wither to the tip of the cantle - how many inches?
But as the others have said, if it doesn't fit the curve of your horse's spine , then when you put you weight down there will be a lot of pressure in the wrong place.
There should be a gap of two fingers width between the underside in the valley and the horses spine.
The saddle should sit parallel to the ground
The angle of the underside should match the angle of the horse's back sideways.
The saddle must not extend back beyond the horse's ribcage
But the big test is whether it leaves marks on the horse's back - if it doesn't fit then: first the hair will go, then a bruise will appear and then a blister.
Not disregarding the possiblity that the horse will get mighty upset Look for, find and discuss with an experienced english rider whether the saddle fits. If you have bought a saddle which doesn't fit your horse, then it is not worth the postage. It could cost you a lot in vet's bills.
At least that Western saddle probably fitted even if you have to stuff an inch thick woolly blanket between it and the horse.