Ben Levi,
You did provide a theory. You said that every form of life was created in the span of one week less then 6000 years ago. You also said that the sizes of animals changed about 4200 years ago (Mabul).
Evolution is a theory that species descend from one another and all life on earth has descended from earlier life forms over billions of years going all the way back to a primitive common ancestor.
The question is which theory is better supported by what we now know about the earth and life.
The fossil record clearly supports the second theory. So does a nested hierarchy, so does genetics, so does the distribution of species etc etc.
It’s that simple.