Just my observations and a little info:
Egg size change is so slow that it is almost unnoticeable. Their 1st few eggs can be quite dramatically weird, however. Tiny, odd-shapes, strangely pigmented, big double-yolkers . . . after awhile, they settle down and become more consistent.
The development charts for leghorn-type birds show that maximum feed consumption doesn't come until 30 weeks for these quick-maturing birds. Full body weight isn't until 60 weeks. So, they are eating most at 30 weeks and continue to gain weight until they are over 1 year old. I think the size of their eggs reflect that maturing.
Pullets have more "normal" eggs after they pass 6 or 7 months of age but the egg size continues to increase until they are over 1 year old.
I was comparing my Australorp & Barred Rock eggs to "large" eggs from the supermarket the other day. The hens are about 16 months old. Their eggs are quite a bit larger than "Large" so I guess they are all Extra Large or Jumbo's these days. If they went from Small to X large or Jumbo - that's a 50% or greater increase in size:
egg sizes, weight dozen
Jumbo, 30 ounces
Extra Large, 27 ounces
Large, 24 ounces
Medium, 21 ounces
Small, 18 ounces
Steve
and we were wondering how long it would take for their eggs to reach the large size? 
