Herd dynamics are a lot more complex than people think. One thing for certain size has nothing to do with it!
Many moons ago we had a big mare that beat up all others. She was nasty in the field. One day a girl turned her pony out with her by mistake. Immediately the big mare went into attack, I will never forget that pony reacting far faster and harder. She beat seven bales out of the big mare. heels teeth and body language. After that the pair were together most of the time and the big mare a far more trustworthy to turn out with anything.
Not the only time I have seen it happen.
If you had watched the herd when I had my old mare, you would have said she was last in pecking order. When feeding the herd - mares with foals at foot and yearlings, she was always the last to go to a feed bowl. She would just wait and then foals would leave their mothers and come eat with her.
Then when the other mares finished they would not go near her, if they did all she had to do was give 'that look' and they would turn away yards before they got anywhere near her.
One day the mare you would have called alpha, first to the feed, always in the lead, must have done something really bad because when it came to feed time, the old mare would just keep her moving away from the feed bowls. No kicking or biting, just a determined 'you are not going to eat' manner.
There was no resistance from the boss mare she just rushed away to chase another from its feed, but that never worked because she was just moved on again!
We had a mare come out of race training and in foal. She was in very poor condition and refused to eat any hard food no matter what and was never really grazing either., she either walked the fence or just stood there.
I put my mare in with her and although my mare was only three or four at the time she would encourage this mare to eat. They became good pals without being 'married' .
The mare went off to stud to foal and be covered again. I get a call from the stud that she would not eat anything and was very poor and not producing milk for the foal.
I took my mare up there and put them in a large pen together - with the foal. Apart from the initial guarding of the foal the mare they were soon all eating from the same feed bowl - interestingly, my mare would almost pretend to eat taking only a few crumbs letting the other eat most.
They stayed together for about 6 weeks and then returned home. The brood mare never refused to eat again.
I do not know what it was about this mare, you could say she was a passive leader but when studied it was more complex than that because she proved, more than once to be boss without teeth and heels.