Using varied sizes of a two-dimensional shape is often an effective way to visually compare quantities. One might think that scaling an object 200% in Illustrator would cause its area to become twice as large. This is not the case. It will actually result in an area four times as large, due to the fact that both the linear height and linear width are scaled by 200%.
Here is a simple formula to calculate the necessary scale percentage, based on the relationship of two values.
Example: We need one square to represent 100 people and another to represent 200 people.
Now just take a copy of the first square and scale it 141.4% in Illustrator.