Is it common? my answer is NO...I had an egg like that ½ size smaller than normal pigeon eggs and it is not fertile...I waited for 5 days and candle it, finds out its not good so I threw it away...But yours might be, so try to wait for a little while and see if its fertile...
I'm not holding out hope for it to be fertile, I doubt it could be it is sooo small. I haven't taken it away yet because they have been sitting so dutifully and the second should be coming this afternoon which I am hoping will be normal size.
About a month ago she had a normal size single egg but at the time her and her mate were having a hard time holding onto their nest box and they abandoned it after a few days when the second egg never came.
Normally Small eggs are associated with very young birds or very old birds. Most of them will not have a yolk sac and will not hatch. I have found there is nothing to worry about, unless it happens consistently, then you have a vitamin problem.