Food is energy. Mares, being the defenders of the herd members and babies (from predatours and hunger), will literally vacuum a yard for the sake of maintaining reserves in case flight is warranted. This is something that is in their instinct box and not going to dilute any time soon.
It could be that her body does work like ours in that if we do’t meet the needs, we can get fluffy even tho we are hardly eating anything. If the right nutrients are not present, a body will subsist on itself. It will reserve and hold onto whatever is available to survive on. Humans actually can GAIN weight from not eating enough..I don’t know that this is true of horses.
Sure there's right and wrong but mostly there's just a whole lotta different.