I've just been writing it off as hay belly. I accidentally came to own this mare and didn't know whether I was going to euthanise, rehome, or keep her. It took 1.5 months to figure out ethanasia was unecessary (working with vets), this last month she was finally assessed by my trainer and that kind of sealed it that I tentatively may be keeping her. But she's been gaining weight all in her belly and your symmetrical comment has me worried. I snapped these pics of her yesterday morning and night.
Both Magic and Whisky needed to gain muscle and weight when I brought them home, so I've been doing the same feeding and supplements that I got the best results with, all feed and supplements are calculated to get each of my mares to their best weight (calculated by me and fine tuned by my vet). They are doing nothing for this mare, every bite of food goes straight to her belly. But she appears to be a QH/TB cross and I've seen TB's gain weight in strange ways but this is a little much.