Understood...it was just info that sometimes to some is not too obvious. It is logical that if there is one cup of water in an incubator that adding another cup will double the humidity but it is not the volume of water that determines the humidity, but the evaporation rate which is controlled by surface size. So you can increase the humidity by taking the same cup of water and putting it in a soup plate instead of a cup.
I'm probably just a little obsessed by this as I set my first batch of eggs in my first incubator last night. I'm doing a dry hatch which I think is a misnomer as the humidity in my house right now is 97 percent...so inside the incubator has to be at least that, and probably much higher. I have to go buy a hydrometer. But...a hydrometer is a little redundant to the hen sitting in the coop on her eggs that has the same ambient humidity.
All I know is that on lockdown a large air sack is a good thing but like you I'd worry about one that big so early in the hatch...and that if there is condensation on my viewing window then that is too much humidity.
I read something about them "drowning" and have to research that. That sounds to me like the air sack is too small when they pip...but I don't know.
Will upping the humidity cause the air sack to shrink? I would not think so...it would just slow down or stop the enlargement wouldn't it?
I use the very unscientific method of judging the humidity in my house and therefore my incubator by checking how fast, or if...my dishrag and dishtowels dry. If they don't dry, it is high.