1:3.5 only means 1/3.5th in one dimension (a line). In two dimensions (a square), it would be 1/3.5^2.
Lets imagine that you have a 4x4 square, and you want to make a 1:2 scale copy of it.
The way you are currently thinking, you are saying, "ok, the 4x4 square has an area of 16, so a 1:2...