The mare sounds as though she is a big-bodied lass to begin with. I like the bold-bodied ones, more confidence in that they won’t break when I ride : ) Any pics? I love pics : )
Questions for your vet might be just as you have presented here. Ask about ration balancers for diet supplementation. We have learned much about diet over the decades, and most grain/feed companies will have some sort of ration balancer available. Mostly, these are a vit/min based mix and fed out in small doses, instead of grain, along with graze/hay. Many horses today are overfed with grains and we are slowly recognizing and acting on the need to feed better, more efficient products.
Best of luck to you with this, it will be interesting to learn how your vet chooses to handle this, please keep us posted!
Sure there's right and wrong but mostly there's just a whole lotta different.