Its possible...usually when a chicken starts laying, the egg will start out smaller, and gradually get to a normal size when they start laying regular aside from the occasional jumbo double yolker...and, bigger chickens will lay bigger eggs to begin with...do you ever catch any of them in the nest box? If you really want to know who is laying what, you could just check frequently, see who is in the box, when there is an egg, take it away, and just keep checking to see who is taking their turn...I know that's time consuming, but if you really want to know for sure who is doing what...we had to do that over a period of 2 weeks when it was time to decide who to get rid of and who to keep when we had to downsize, as I wanted to be absolutely sure who was regularly using the nesting box to lay eggs, and we banded those we wanted to keep, but it was worth it, and I think we picked the right ones as we are still getting the eggs we should be.