No one could be fatter.No one could have been fatter.意思上有什么不

No one could be fatter.的意思是“不会有人比（他）更胖”讲的是现在的情况； No one could have been fatter.的意思是“（当时）肯定没有人比（他）更胖的”,表达的是过去的情况.