Someone gave me a couple of duck eggs to incubate, but I have no idea the breed. They are white, and about to same size of my brahma chickens' eggs. Anywho, I am on day 21 and I candled. The entire egg is filled (except air sac) and I can barely see movement because it's filled so much. My concern is the air sac seems very small compared to pictures on the internet and compared to when I hatched chicken eggs. It looks like it only takes up about 1/6 of the egg. Should I be concerned? Do smaller eggs generally hatch faster? I wouldn't think the embroyo would be so large at 21 days.