The tightness of mares depends on many factors. Of course, first of all, it is her size and breed. (A heavy draft mare will be much freer than the same English thoroughbred or Arabian, I won't even mention ponies and mini ponies).
But there are also additional factors such as whether the mare has given birth and how many times (naturally, a mare with a 10th foal on the way will not be tight) and no less important is the factor of the general condition of the horse, the more "athletic" she is, the more developed her muscles are, the tighter she would be.
Although we should not forget about individual characteristics, there are those who are tight from birth and those who are not given it.
I can judge by my mare, before me she was a virgin and except for me she had no one, she is of the Arabian breed and I can say with full responsibility that she is tighter than most of the human girls I have had, especially considering the fact that as she approaches orgasm/arousal, she tightens even more.