My daughter recovered a medium egg yesterday. According to some reading that I've done, new layers will do exactly what Mildred is doing - being erratic in the type of eggs produced, laying all sorts of sizes until settling in to consistency. Hence the extra large or jumbo double yolkers.

I've also seen some speculation on the Ross breed stating that they may not reproduce well, but based on the literature I downloaded from Aviagen, the Ross is supposed to be a competent breeder.

More observation and research. And the search for more hens continues.