Smaller eggs of the same breed tend to hatch a little earlier than larger eggs of the same breed. That I am convinced of. I do not know if bantam eggs tend to hatch earlier than regular eggs. I have no experience with bantam chickens.
What I mean by tend to hatch earlier is that if you have a lot of eggs, you'll notice a difference. But if you only look at a very few eggs, it does not always work. For averages to mean anything, you have to have enough samples.
On my first 2 hatches that had Bantams they hatched first ~ this time my FBCM, EE and Ameraucana have all started hatching, but no Bantam Cochins yet. They started hatching overnight last night, but aren't actually due until tomorrow.
I ususally set both Bantam and regular sized eggs in each of my hatchings. The bantams always hatch first--usually the day after lockdown. This time I have only bantam eggs, so i am making lockdown day a day earlier.
I think every time I've read of bantam hatching eggs hatching in less than 21 days it involved an incubator. Since I almost exclusively let my hens do the work my hatch days are always right at 21. I have had a couple pip early but not hatch until they were due.