I'm usually into larger mares, but I've seen some ponies and mini mares with lovely butts. It would be quite an experience to have sex with a mare where I wouldn't need a box or a pedestal to stand on just to line my penis up with her parts, but the smaller the mare gets, the less defined her butt looks.
I remember seeing a video of a heavily gravid pregnant Shetland (foaling instruction video), where she was setting herself down to roll onto her side to begin foaling. It was during the transition between standing and lying down where I saw the pregnant mare's udders peeking from behind her, and I was legit attracted and aroused by the sight; when she drops to her forearms and her ass is still in the air and you can see those udders poking out behind her from between her thighs.
To be honest, I feel guilty when I think about sex with smaller mares. I feel like it borders with being full-blown pedo.