Why are larger hex mirrors round and the smaller aren't? Per this diagram why are the larger (30 meter and European) mirrors filled out to nearly a full circle, while the smaller mirrors composed of hexagons retain a hexagonal border limit instead of filling out and approximating a full circle?
Start by  drawing the largest inscribable circle (circle completely contained within the mirror assembly).  Then  draw, e.g., the smallest superscribing circle (completely contains all the mirrors), and calculate how many more hexagons are needed to fill the additional area covered.  

Finally, draw the circle showing the design entrance aperture of the telescope to see how much of the aperture is lost vs. how much of the outer hexagons' area is unused in each case.   

Then do a cost vs.  quality tradeoff.  Take into consideration (for the smaller telescopes) the delta cost of using more, smaller hexagons.