I am reluctant to advise without seeing a picture. Usually when someone says they have a fat yearling mini they are actually too thin which causes the tummy to bloat way out. My bet is she's not getting enough protein.
Be sure and feel her with your hands. If you can feel the backbone but she has a big belly she is lacking protein.
I know its hard to believe but 9 times out of 10 if you feed them more and a better feed (16 percent) the belly disappears.
You have to be careful working yearlings as they arent done growing and you can damamge joints
^^^I second this as we had a yearling that everyone thought was fat but was really lacking protein and needed weight, often it is hard to get them through this stage if they are really bad. I had one it took us a year or more to get completely out of this that came from an auction.