My children and I did eggtopsies on our eggs that didnt hatch. We waited about 5 days after the last chick hatched in the incubator, candled them and did the float test. After both tests came back with neg. results, we opened them. I have 1-2 that were like that size along with a couple that didnt seem to be fertilized at all and a bunch of fully developed chicks that must have drowned :0( There was one chick that pipped through his/her air sac but then did nothing else AND one chick fully developed and HUGE but didnt have an air sac at all! That was weird, Im not sure what would have cause that...
we did this in our animal science class last year to about 20 eggs that didnt hatch.
we got like 4 that looked like yours in the picture, 4 almost full grown chicks that had died before they hatched, and then about 8 that were like barely there blobs, and the rest were non-fertilized.
pretty interesting. some had bacterial rings that killed them, others looked like healty birds that died in unobvious ways.