Why is there an "indent" in my horse's backside?

We have had 2 horses, Spike and Reba, for a few months now. Recently a farrier has came out to shoe them and noticed an "indent" on Spike's hips. I know he is not being underfed, in fact we have upped his grain intake. They both get a scoop of grain in the morning and night and about two or three leaves of hay twice a day. We didn't notice anything until the farrier said something. Nothing has changed behaviorally, and Reba does not have it. It is more present on the left side, facing his , but increases or decreases depending on his stance. It starts just a few inches past where the saddle ends and is circular. It does not extend down to his thigh and remains mostly on his back. We wormed them a few weeks ago and he had diarrhea one day, if that means anything. It really is bothering us and want to make sure he is the happiest healthiest horse he can possibly be (both of them), so if you have any ideas whatsoever or sites where I could go that might give me an answer I would greatly appreciate it!