Ummm... there's not a lot of difference among these words. It's certainly not the case that you could take any two of those words and say A is bigger than B. However, some of these words are more often used to describe 1-dimensional and 2-dimensional spatial "bigness" and others are more often used to describe 3-dimensional "bigness". So... colossal, gigantic, huge, immense, titanic more often describe 3-dimensional "bigness" whereas vast tends to describe either a 2-dimensional area (prairie, desert) and 3-dimensional volume (outer space). CAVEAT: "More often" simply means more often, but not always. So... one will often see mention of an "immense prairie" or a "huge desert". There simply are not hard and fast rules on how to use these words.