Because for a fraction with a small denominator, you don't have to split them up into small pieces and as many pieces. For example: let's say you had 2/3. you draw a circle with 3 equal quadrants. Those quadrants are big because you didn't have to split the circle up into a lot of peices; only 3.
Now for a bigger denominator like 20/32, you have to split the circle into 32 peices. For all of them to fit, you have to make the pieces small.
