Weird, I thought for sure that had something to do with various 4chan OCs (parallel “manlet”), but there’s no evidence of it. I would interpret it as a tag for growth-stunted adult ponies, whereas “smol” might apply to normally-developed ponies that are just of small stature (how would you tell the difference? I dunno), non-adult ponies, and off-model artistic choices that depict a character being small.
Yeah, I saw those a while ago, too. And yeah, short is more about leg and neck length while smol is whole body size. I’d pretty much left it alone for those who care about it to sort out.
In my head, tiny ponies are innocently cute, just for the fun of it, while micro is more blatantly someone’s special interest (not necessarily sexual, but you can somehow tell it’s more than “aww, a very little pony!”) I mind seeing micro, and I do not mind seeing tiny ponies, and I have a hard time specifying what makes the difference. I would compare “feet” vs. “foot fetish”.