Can I ask if youre having this investigated ? Id be trying to find out why the uneven muscle development rather than using a pad to solve the problem.
I used a korrector pad on my old horse when she lost muscle tone along her back and was still in ridden work , it was a fantastic piece of kit to help where she was losing muscle on one side due to her gait, which had changed due to her arthritis.
They don't add a lot of bulk at all and mine was a Godsend......I have a 21yr old WB ex Showjumper, and he came to me in very poor condition so the Korrector pad was a great shock absorber and was great for that extra bit of balancing whilst uilding uneven muscle....once the muscle came b ack I kept it on my saddle as an all round comfort pad.
It has been investigated at great length. I am not using the pad as a quick fix. Few horses are perfectly even in muscle developement. Her right shoulder is slightly smaller than her left and it tends to tip the saddle and me to the right. I was told to pad it slightly more until the shoulders have evened out with exercise.
I've just ordered the Aerolite Combi Riser for my boy (after discussions with vet/ROR consultants). He's an ex racer and has no topline at all and his spine is very visible ... we've got him the high wither T4 saddle with interchangable gullet but when sat on it's still riding a little low on his entire back ...