David's embryos

I just took it at face value. Actually something that size wouldn't have even fit and pass that easily from his throat. 

I honestly never considered it until you brought it up.

I assumed that David knew the method and dimensions of the Covenant's storage mechanism. He's got full access to MUTHUR and that information would be available.

Probably that size and shape that contained the embryos and the incubators were standardized for many years, not specially designed for the Covenant. Not sure if the facehuggers embryos needed the incubator, that scene was used most probably as a metaphor: a deadly predator next to the human embryos who represented the future of the mankind. Like the wolves guarding the sheep.

@BlackGooDrinker - I agree with dk, as I never thought about it until you just mentioned it, too?  I am led to assume Weyland/Yutani have more in 'development' that we are aware of?  I wonder if these colonists are indeed clones?  Interesting observation on your part!