It's all those dang 5s: 3.14159265358...Sorry, but @kaihp is correct, a 600/5.6 entrance pupil is ~14.5% larger than that of a 400/4.
You might want to revisit some basic principles of geometry that you probably learned as a youth. I’d suggest starting with the method for calculating the area of a circle. A pi may help your calculations, but please don’t try to eat it (irrational numbers are notoriously hard on your digestion).
Well, we've had irrational posters, why not irrational numbers?