I have only just started to keep ants this month But with the research I've already done I've seen that normally only the smaller ant kinds ever have multiple queens and its rare for the larger kind to have more than one queen. My first thought on that ep of his videos was just that. And also because they were larger and not raised together to start off, I would think they won't merge into one but instead do what they did. I'm just your average person that has done some research, and if I could come to that conclusion so easy why can't a many years ant keeper do the same. I've also been watching AC for a few years every now and then and have hated how his videos have gotten "climaxed" and the way he speaks has changed. I love his old videos with his normal voice and general love for the ants.
I don't think size actually affects whether or not the species is polygynous, but being small seems to be common in polygynous ants. At least the species that I know of.
There are polygynous harvester ants here in Southern California, and they are pretty big ants.