You are a fat pig或者big pig其实都无所谓,因为这个句子本来就不是强调语法的.
另外如果你要说那个人笨的话可以用
You are such an idiot!你真是一个白痴.
You are a stupid/You are so stupid.你是一个大笨蛋
You are a dumbass.你是一个蠢蛋