Dave that's so awesome!!! Please continue to post your thoughts of embryo development and keep us updated, as this is so unique...how fun and exciting for you and all of us to experience with you!!! YAY!
The big part of the egg is about the same size as all of the others, it just has the added smaller bulb. Maybe 1.5 times the size as the other eggs?
We took my female into the vet before she laid and got this which is pretty neat to see as well. I thought it was so cool that I went back to the vets today to take a picture of it.
My friend, Shelly, had a Russian egg a couple of years back like that. Felt sorry for the poor female laying it. Unfortunately it did not hatch out. I know she was going to take it to school and let her students cut it open (after is way long over due to hatch). I keep forgetting to ask her, what they did find upon opening it. Good luck with yours.