I had a similar concern with egg size in one of my hens. I kept all seven hens from my June 2012 Sandhill Reds. Three are not good quality birds, but lay a beautiful, extra large egg with a little too much tinting. They sell quite well, and will never be hatched. Of the other four that are fairly nice birds in various ways, one is amazing. Her frame is exactly what I want, she's over 7 lbs, her feet and head are nice, she's got a wonderful personality (I know, not an SOP requirement, but important to me), and she has all the appropriate farm behavioral instincts intact (again, not in the SOP, but I need utility farm birds that thrive with free-range management). Her color is a little more CD than RD, but I don't care at this stage in my breeding program. But her pullet eggs were too small. Honestly, I wasn't even sure that a chick could survive in one, as they were only market medium on a good day. She started laying in November 2012 and continued through July of 2013 with very little increase in egg size. But her eggs this year are excellent. Large size, ideal shape, good shell porosity, normal shell strength, and classic ivory white color. I don't know if the shift in egg size at year two is a genetic issue with her, or if she had a medical issue that has resolved, but if her daughters do the same thing then that will tell me. It's not a desirable trait, but given her other qualities she'll still be one of my foundation hens. Have your hens with small eggs gone into their second year yet?
I have found most of my Faverolles girls (close cousin to Dorking as Dorkings are one of the foundation breeds of Faverolles) always increase in egg size going into their second year. A long time Faverolles breeder recommended to me to not breed until the girls were a minimum of 11 months, partially to allow time for the egg size to come up, but more importantly to allow true evaluation of mature structure before selection for breeding pens.