Oh dear.....They COULD be being laid by an older hen that is having some sort of mishap in her egg machine. I think they're as cute as can be but I prefer not to find them... for that reason. Not sure what the problem is and have no idea what to do about it.
I'm sure its possible that if a bantam laid a really small egg it may have a yolk in it but when the come from a large bird? Sometimes, too, when they first start laying again after a molt I may get a tiny egg like that but usually only one. I'm thankful that their systems get up and running properly in short order!
I hope I don't have a sick hen....