Here is an earlier F-18 model... 
with round intakes, so I assume the larger, rectangular intakes allow more air to enter the engines.
What is the advantage of having rectangular intakes? Any specific reason?
Round intakes would stick further out, reducing the effective wing area.
The number of square inches is what is important. The actual intake to the engine is quite a ways back in the body of the airplane.
Here is an earlier F-18 model... with round intakes, so I assume the larger, rectangular intakes allow more air to enter the engines.
Reduced radar reflection, 18% increase in air flow as compared to the D-shaped intake on the Hornet and “better” high speed performance.
redurced radar signature