I think a certain amount of chair seat is unavoidable if the horse is so wide that it would force your hips into a very unnatural position to try and force the leg to be more vertical. That's why when people look at a wide horse and say , "he must be really comfy to ride, he looks like a pillow" , they are wrong. A really wide horse is often much less comfy to ride than a more narrow horse.
You look fine on him and if he is not objecting, then just ride.
BTW, where was that photo taken? (I am from WA )