Dear MG: Full as an egg is not really an impressive metaphor. Have you never noticed the rather large air sac in one end of the egg? At the moment I can't remember which end for sure, but I think it is the big end.
If an egg has about thirty cc. of contents, almost two cc is in that little sac. I suppose it is necessary so that rise in temperature which cause liquid volume to increase can't cause rigid shell to break open.