These are questions I haven't been able to find an answer to...

[SunnyChic]

Sometimes the fresher/newer the egg, the smaller the air sac. If you candle a store bought egg (I practice candling with them), they usually have a larger air sac. As the egg ages (or incubates), moisture is lost within the egg, making the air sac gradually get larger. Here is a pic of some shipped eggs I received; I candled and outlined the air sacs in pencil. They will vary from egg to egg.

Here's a link that has helped me:
http://lancaster.unl.edu/4h/Embryology/embryophotos.shtml