Help me in custom models

I am making a mod where it has a set of armor but i wanted that armor to be skin tight not bulky so i searched and found a youtube tutorial about how to make custom armor models so in the tutorial he only made a hat and when i tried it it worked but when i tried to follow the same method for the chestplate but the arms become the chestpate but small i tried many ways to fix this but i dont know how.