I echo what Iron Maiden and Spiggly have said. If a saddler is recommending a narrower saddle, it's certain that the horse has lost muscle and the saddle has been the probable cause and if you follow that route, you will be making matters much worse. In actual fact what it possible is that your horse may have needed to move to a wider saddle some time ago as the muscle loss suggests that the current saddle may not have been wide enough. Work encourages increased muscle, not decreased muscle and those hollows and dips seen around many withers are not a sign of fitness or loss of fat.
Ground work is very underated and it would be time well spent and money saved if you could build your horse back up before saddle shopping. Horses need good healthy back muscle to support a saddle and the weight and pressure we riders transmit through that. Any restriction will result in reduced blood flow which means reduced oxygen to muscles when they most need it which starts the downhill spiral resulting in atrophy. I do beleive that many dipped backs are the result of ill fitting saddles preventing the horse from using themselves as nature intended.
Ground work is very underated and it would be time well spent and money saved if you could build your horse back up before saddle shopping. Horses need good healthy back muscle to support a saddle and the weight and pressure we riders transmit through that. Any restriction will result in reduced blood flow which means reduced oxygen to muscles when they most need it which starts the downhill spiral resulting in atrophy. I do beleive that many dipped backs are the result of ill fitting saddles preventing the horse from using themselves as nature intended.