If you postulate one idea (form) to account for the fact that all big things are big, you are still left with two classes of big things -- the idea and the particulars --- and you still have to explain why both those classes of big things are big. You will then have to have a further idea for both classes to participate then, and then in the same way a third idea to account for the fact that the first and the second idea have a common predicate, big, and so on ad infinitum.