Originally Posted by clippityclop
Worst case scenario but one to consider is muscle atrophy. It can be caused from an ill fitting saddle - either too narrow or in some cases, too wide - here is some info showing a similar horse and his is due to the saddle being too wide... (scroll to the pic of the sorrel at the bottom). The Trapezius muscle
If the muscle is atrophied, it can be strengthened and can grow bigger. If it is damaged, then it may not. When we lay in bed for six days our muscles atrophy. When we get kicked by a horse, the muscle fibers can be torn and permanently damaged.
That horse on the bottom photo is also too thin. His topline may look better with more groceries.
The OP's horse also looks too thin. A saddle did not create the hollow between the two sides of the croup, the depression in the hindquarters or the elevation in the spot where horses get "hunter's bump." Those along with the elevated spine along the back show that the horse is underweight.
I have had good success with getting proper muscling over the back and neck by riding up hills and down hills.
Unless a horse is very old, their spine should be flat across. If it rises above the back, they are too thin (sometimes just a touch). If it makes a channel depressed in the back, the horse is too fat. A general rule of thumb.