Mine does. The pictures aren't the greatest to show how much difference there is, but the amount of muscle that was lacking versus how much is there now has made a significant difference in my saddle's fit. The before picture I had to use a much thicker pad and now it's almost too tight for her.
Before: Sometime last winter. I have a circulation disorder that makes my hands and feet very sensitive to cold, so with the winters here being so freezing, I don't ride much then, so she loses a fair amount of muscling. Notice how bony her withers look and that the outline of her shoulder blade sticks out.
Now: She spent all summer in a large pasture with hills and exercised herself on them quite a bit because the water source was completely across the pasture from the main gate. This built up a fair bit of muscle and I've been trying to maintain it.
Our mare was underweight and hadn't received any substantial training beyond basic ground manners when we got her. She's now had about 6 months under saddle (plus some extra groceries, quality feed supplements, and regular hoof care)
KayceeJo, you're right! She's a mustang. Her name is Eva, and she's originally from the Oregon range. She is one of the Kiger mustangs if you're at all familiar with them. We adopted another mustang at an event sponsored at the Oregon BLM facility and Eva was adopted at the same time by the lady who helped us transport and gentle our colt.
Eva's training was somewhat sporadic and she was passed around a bit during her first year as a domestic horse. My family and I were ultimately contacted by the original adopter to let us know that she was being given away. Knowing that she was a "diamond in the rough" and fearful that whoever else took her might not discover her true potential, we took her in. It was one of the best decisions we ever made!