I haven't read the other replies, but I will offer this. I am currently feeding my lactating mare two full scoops of nutrena prime every day. (One at breakfast and one at dinner.) It seems to be the perfect amount to keep weight on her while she's nursing. Her demand may increase or decrease. Prime is a 12% protein feed. I would be hesitant to feed minis a higher percentage of protein than that.
Also, she doesn't get much grass, but she does get grass hay. Even if she were to be getting grass, she may still need to be grained. Remember to change amounts slowly, but she probably does need grain. My girl is a good weight, even with a belly they can be too thin. Mini mares like people just tend to get stretched out from pregnancy. Out of an entire herd that would be getting grass or just grass hay a lactating mare would demand grain the most.