There are two things you have to consider while fattening up a pig. Firstly, if the pig is still young and small, keep on pig starter and let them consume as much as they want to. This feed is rich in carbohydrates and will allow your pig to grow big. However, once they reach about 150 pounds, hold back on the pig starter. It's mostly fat, and the weight that your pig will gain from it will be fat as well. You can swap to a flavor-enhancing supplement and add it to your pig's food. Also, don't forget to make exercise a regular part of your pig's routine. Exercise will reduce fat and give much more room for muscle development.